Semiconductor device

ABSTRACT

Provided is a structure of a transistor, which enables a so-called normally-off switching element, and a manufacturing method thereof. Provided is a structure of a semiconductor device which achieves high-speed response and high-speed operation by improving on characteristics of a transistor, and a manufacturing method thereof. Provided is a highly reliable semiconductor device. In the transistor in which a semiconductor layer, source and drain electrode layers, a gate insulating layer, and a gate electrode layer are stacked in that order. As the semiconductor layer, an oxide semiconductor layer which contains at least four kinds of elements of indium, gallium, zinc, and oxygen, and has a composition ratio (atomic percentage) of indium as twice or more as a composition ratio of gallium and a composition ratio of zinc, is used.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method formanufacturing the semiconductor device.

In this specification, a semiconductor device generally means a devicewhich can function by utilizing semiconductor characteristics, and anelectrooptic device, a semiconductor circuit, and electronic equipmentare all semiconductor devices.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate having an insulatingsurface (also referred to as a thin film transistor (TFT)). Thetransistor is applied to a wide range of electronic devices such as anintegrated circuit (IC) or an image display device (display device). Asilicon-based semiconductor material is widely known as a material for asemiconductor thin film applicable to a transistor. As another material,an oxide semiconductor has been attracting attention.

For example, a transistor whose active layer includes an amorphous oxidecontaining indium (In), gallium (Ga), and zinc (Zn) is disclosed (seePatent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528

SUMMARY OF THE INVENTION

One object of the present invention is to provide a structure of atransistor including an oxide semiconductor in a channel formationregion in which the threshold voltage of electric characteristics of thetransistor can be positive, which is a so-called normally-off switchingelement, and a manufacturing method thereof.

Further, another object of one embodiment of the present invention is toprovide a structure of a semiconductor device which achieves high-speedresponse and high-speed operation by improving on characteristics of atransistor (e.g., on-state current or field-effect mobility), and toprovide a manufacturing method thereof, in order to achieve ahigh-performance semiconductor device.

Further, another object is to provide a semiconductor device in whichreliability is high and threshold voltage does not easily shift even ina long-time use.

It is an object of one embodiment of the present invention to achieve atleast one of the above-described objects.

In a transistor in which a semiconductor layer, source and drainelectrode layers, a gate insulating layer, and a gate electrode layerare stacked in that order. As the semiconductor layer, an oxidesemiconductor layer which contains at least four kinds of elements ofindium, gallium, zinc, and oxygen, and has a composition ratio (atomicpercentage) of indium as twice or more as a composition ratio of galliumand a composition ratio of zinc, is used.

One embodiment of the present invention disclosed in this specificationis a semiconductor device including an oxide semiconductor layerincluding a channel formation region over an oxide insulating layer, agate insulating film over the oxide semiconductor layer, and a gateelectrode layer overlapping with the oxide semiconductor layer over thegate insulating film. The oxide semiconductor layer contains at leastfour kinds of elements of indium, gallium, zinc, and oxygen, and has acomposition ratio (atomic percentage) of indium as twice or more as acomposition ratio of gallium and a composition ratio of zinc.

One embodiment of the present invention disclosed in this specificationis a semiconductor device including an oxide semiconductor layerincluding a channel formation region over an oxide insulating layer, asource electrode layer and a drain electrode layer over the oxidesemiconductor layer, a gate insulating film over the source electrodelayer and the drain electrode layer, and a gate electrode layeroverlapping with the oxide semiconductor layer over the gate insulatingfilm. The oxide semiconductor layer contains at least four kinds ofelements of indium, gallium, zinc, and oxygen, and has a compositionratio (atomic percentage) of indium as twice or more as a compositionratio of gallium and a composition ratio of zinc.

One embodiment of the present invention disclosed in this specificationis a semiconductor device including a pair of first oxide semiconductorlayers separated from each other over an oxide insulating layer, asecond oxide semiconductor layer including a channel formation region onand in contact with the oxide insulating layer and the pair of firstoxide semiconductor layers, a gate insulating film over the oxideinsulating layer and the second oxide semiconductor layer, and a gateelectrode layer overlapping with the second oxide semiconductor layerover the gate insulating film. The second oxide semiconductor layercontains at least four kinds of elements of indium, gallium, zinc, andoxygen, and has a composition ratio (atomic percentage) of indium astwice or more as a composition ratio of gallium and a composition ratioof zinc.

The oxide semiconductor layer or the second oxide semiconductor layer isa non-single crystal semiconductor, and may include a c-axis-alignedcrystal region.

The oxide semiconductor layer or the second oxide semiconductor layer isa non-single crystal semiconductor, and can be formed with an oxidetarget having a composition ratio of indium:gallium:zinc=3:1:2.

In the oxide semiconductor layer or the second oxide semiconductorlayer, a region which does not overlap with the gate electrode layer mayinclude a dopant.

In the oxide semiconductor layer or the second oxide semiconductorlayer, a region which does not overlap with the source electrode layeror the drain electrode layer may have a higher oxygen concentration thana region which overlaps with the source electrode layer or the drainelectrode layer.

Low-resistance regions whose resistances are lower than that of thechannel formation region and which include a dopant may be formed in theoxide semiconductor layer so that the channel formation region issandwiched between the low-resistance regions, by introducing the dopantinto the oxide semiconductor layer in a self-aligning manner with theuse of the gate electrode layer as a mask. The dopant is an impurity bywhich the electrical conductivity of the oxide semiconductor layer ischanged. As the method for introducing the dopant, an ion implantationmethod, an ion doping method, a plasma immersion ion implantationmethod, or the like can be used.

With an oxide semiconductor layer which includes low-resistance regionsbetween which a channel formation region is sandwiched in the channellength direction, the transistor has excellent on-state characteristics(e.g., on-state current and field-effect mobility) and enableshigh-speed operation and high-speed response.

Further, heat treatment (dehydration or dehydrogenation treatment) bywhich hydrogen or moisture is released may be performed on the oxidesemiconductor layer. When a crystalline oxide semiconductor layer isused as the oxide semiconductor layer, heat treatment forcrystallization may be performed.

Through the dehydration or dehydrogenation treatment, oxygen that is amain component material of an oxide semiconductor might be detached andthus might be reduced. There is an oxygen defect in a portion whereoxygen is detached in the oxide semiconductor film and a donor levelwhich leads to variation in the electric characteristics of a transistoris formed owing to the oxygen defect.

Thus, oxygen is preferably supplied to the oxide semiconductor layerafter being subjected to the dehydration or dehydrogenation treatment.By supplying oxygen to the stack of oxide semiconductor layers, oxygendefects in the film can be repaired.

For example, an oxide insulating film including much (excessive) oxygen,which serves as an oxygen supply source, may be provided so as to be incontact with the oxide semiconductor layer, whereby oxygen can besupplied to the oxide semiconductor layer from the oxide insulatingfilm. In the above structure, heat treatment may be performed asdehydration or dehydrogenation treatment in the state where the oxidesemiconductor layer and the oxide insulating film are in contact witheach other at least partly to supply oxygen to the oxide semiconductorlayer.

Further or alternatively, oxygen (which includes at least one of anoxygen radical, an oxygen atom, and an oxygen ion) may be added to theoxide semiconductor layer after being subjected to the dehydration ordehydrogenation treatment to supply oxygen to the oxide semiconductorlayer. As a method for introducing oxygen, an ion implantation method,an ion doping method, a plasma immersion ion implantation method, plasmatreatment, or the like may be used.

Further, it is preferable that the oxide semiconductor layer in atransistor include a region where the oxygen content is higher than thatin the stoichiometric composition of the oxide semiconductor in acrystalline state. In that case, the oxygen content is higher than thatin the stoichiometric composition ratio of the oxide semiconductor.Alternatively, the oxygen content is higher than that of the oxidesemiconductor in a single crystal state. In some cases, oxygen existsbetween lattices of the oxide semiconductor.

By removing hydrogen or moisture from the oxide semiconductor to highlypurify the oxide semiconductor so as not to contain impurities as muchas possible, and supplying oxygen to repair oxygen vacancies therein,the oxide semiconductor can be turned into an i-type (intrinsic) oxidesemiconductor or a substantially i-type (intrinsic) oxide semiconductor.This enables the Fermi level (E_(f)) of the oxide semiconductor to be atthe same level as the intrinsic Fermi level (E_(i)). Accordingly, byusing the oxide semiconductor layer for a transistor, fluctuation in thethreshold voltage Vth of the transistor due to an oxygen vacancy and ashift of the threshold voltage ΔVth can be reduced.

One embodiment of the present invention relates to a semiconductordevice including a transistor or a semiconductor device including acircuit which is formed by using a transistor. For example, oneembodiment of the present invention relates to a semiconductor deviceincluding a transistor in which a channel formation region is formedusing an oxide semiconductor or a semiconductor device including acircuit which is formed by using such a transistor. For example, thepresent invention relates to an electronic device which includes, as acomponent, an LSI; a CPU; a power device mounted in a power circuit; asemiconductor integrated circuit including a memory, a thyristor, aconverter, an image sensor, or the like; an electro-optical devicetypified by a liquid crystal display panel; or a light-emitting displaydevice including a light-emitting element.

According to one embodiment of the present invention, a structure of atransistor including an oxide semiconductor in a channel formationregion in which the threshold voltage of electric characteristics of thetransistor can be positive, which is a so-called normally-off switchingelement, and a manufacturing method thereof can be provided.

Further, according to one embodiment of the present invention, in orderto achieve a semiconductor device having higher performance, a structurefor improving on-state characteristics of the transistor (e.g., on-statecurrent and field-effect mobility) and for achieving high-speed responseand high-speed operation of the semiconductor device and a manufacturingmethod thereof can be provided.

Further, according to one embodiment of the present invention, asemiconductor device in which reliability is high and threshold voltagedoes not easily shift even in a long-time use can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E are diagrams showing one embodiment of a semiconductordevice and a method for manufacturing the semiconductor device.

FIGS. 2A to 2C are diagrams showing one embodiment of a semiconductordevice.

FIGS. 3A and 3C are diagrams showing one embodiment of a semiconductordevice.

FIGS. 4A to 4E are diagrams showing one embodiment of a semiconductordevice and a method for manufacturing the semiconductor device.

FIGS. 5A to 5C are a cross-sectional view, a plan view, and a circuitdiagram showing one embodiment of a semiconductor device.

FIGS. 6A and 6B are a circuit diagram and a perspective view showing oneembodiment of a semiconductor device.

FIGS. 7A to 7C are a plan view and cross-sectional views showing oneembodiment of a semiconductor device.

FIGS. 8A and 8B are circuit diagrams showing one embodiment of asemiconductor device.

FIG. 9 is a block diagram showing one embodiment of a semiconductordevice.

FIG. 10 is a block diagram showing one embodiment of a semiconductordevice.

FIG. 11 is a block diagram showing one embodiment of a semiconductordevice.

FIG. 12 is an energy band diagram of an oxide semiconductor.

FIG. 13 is a graph showing results of XRD measurement of an oxidesemiconductor film.

FIG. 14 is a graph showing evaluation results of electricalcharacteristics of a transistor 1.

FIGS. 15A and 15B are graphs showing evaluation results of electricalcharacteristics and reliability of a transistor 2.

FIGS. 16A to 16C are TEM photographs of an oxide semiconductor film.

FIG. 17 is a graph showing a leakage current of a transistor.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the invention disclosed in thisspecification are described with reference to the accompanying drawings.Note that the invention disclosed in this specification is not limitedto the following description, and it is easily understood by thoseskilled in the art that modes and details can be variously changedwithout departing from the spirit and the scope of the invention.Therefore, the invention disclosed in this specification is notconstrued as being limited to the description of the followingembodiments. Note that the ordinal numbers such as “first” and “second”in this specification are used for convenience and do not denote theorder of steps and the stacking order of layers. In addition, theordinal numbers in this specification do not denote particular nameswhich specify the present invention.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and amethod for manufacturing the semiconductor device are described withreference to FIGS. 1A to 1E and FIGS. 3A to 3C. In this embodiment, atransistor including an oxide semiconductor film is described as anexample of the semiconductor device.

The transistor may have a single-gate structure in which one channelformation region is formed, a double-gate structure in which two channelformation regions are formed, or a triple-gate structure in which threechannel formation regions are formed. Alternatively, the transistor mayhave a dual-gate structure including two gate electrode layerspositioned over and under a channel formation region with a gateinsulating film provided therebetween.

A transistor 440 a shown in FIGS. 1A to 1E is an example of a planartype transistor having a top-gate structure.

The transistor 440 a includes, over a substrate 400 having an insulatingsurface over which an oxide insulating layer 436 is provided, an oxidesemiconductor layer 403 including a channel formation region 409,low-resistance regions 404 a and 404 b, and low-resistance regions 406 aand 406 b, a source electrode layer 405 a, a drain electrode layer 405b, a gate insulating film 402, and a gate electrode layer 401. Aninsulating film 407 is formed over the transistor 440 a.

In FIGS. 1A to 1E, the source electrode layer 405 a and the drainelectrode layer 405 b are not overlapped with the gate electrode layer401, over the oxide semiconductor layer 403; however, the sourceelectrode layer 405 a and the drain electrode layer 405 b may be partlyoverlapped with the gate electrode layer 401 like a transistor 440 bshown in FIG. 2A.

The oxide semiconductor layer 403 is an oxide semiconductor layer (alsoreferred to as an IGZO layer) which contains at least four kinds ofelements of indium, gallium, zinc, and oxygen, and has a compositionratio (atomic percentage) of indium as twice or more as a compositionratio of gallium and a composition ratio of zinc.

The oxide semiconductor layer 403 can be formed by a sputtering methodwith an oxide target having a composition ratio ofindium:gallium:zinc=3:1:2.

The oxide semiconductor is non-single-crystal and may be eitheramorphous or polycrystalline. Further, the oxide semiconductor may haveeither an amorphous structure including a portion having crystallinityor a non-amorphous structure.

In an oxide semiconductor in an amorphous state, a flat surface can beobtained with relative ease, so that when a transistor is manufacturedwith the use of the oxide semiconductor, interface scattering can bereduced, and relatively high mobility can be obtained with relativeease.

In an oxide semiconductor having crystallinity, defects in the bulk canbe further reduced and when a surface flatness is improved, mobilityhigher than that of an oxide semiconductor in an amorphous state can beobtained. In order to improve the surface flatness, the oxidesemiconductor is preferably formed over a flat surface. Specifically,the oxide semiconductor may be formed over a surface with the averagesurface roughness (R_(a)) of less than or equal to 1 nm, preferably lessthan or equal to 0.3 nm, more preferably less than or equal to 0.1 nm.

Note that the average surface roughness (R_(a)) is obtained byexpanding, into three dimensions, arithmetic mean surface roughness thatis defined by JIS B 0601: 2001 (ISO4287:1997) so as to be able to applyit to a curved surface. R_(a) can be expressed as an “average value ofthe absolute values of deviations from a reference surface to adesignated surface” and is defined by the following formula 1.

$\begin{matrix}{{Ra} = {\frac{1}{S_{0}}{\int_{y_{1}}^{y_{2}}{\int_{x_{1}}^{x_{2}}{{{{f\left( {x,y} \right)} - Z_{0}}}\ {\mathbb{d}x}\ {\mathbb{d}y}}}}}} & \left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Here, the specific surface is a surface which is a target of roughnessmeasurement, and is a quadrilateral region which is specified by fourpoints represented by the coordinates (x₁, y₁,f(x₁, y₁)), (x₁, y₂,f(x₁,y₂)), (x₂, y₁, f(x₂, y₁)), and (x₂, y₂, f(x₂, y₂)). S₀ represents thearea of a rectangle which is obtained by projecting the specific surfaceon the xy plane, and Z₀ represents the height of the reference surface(the average height of the specific surface). Ra can be measured usingan atomic force microscope (AFM).

As the oxide semiconductor layer 403, an oxide semiconductor layerincluding a crystal and having crystallinity (crystalline oxidesemiconductor layer) can be used. The crystals in the crystalline oxidesemiconductor layer may have crystal axes oriented in random directionsor in a certain direction.

For example, an oxide semiconductor layer including a crystal having ac-axis which is substantially perpendicular to a surface of the oxidesemiconductor film can be used as the crystalline oxide semiconductorlayer.

The oxide semiconductor layer including a crystal having a c-axissubstantially perpendicular to a surface has neither single crystalstructure nor amorphous structure and is an oxide semiconductor layerincluding a c-axis aligned crystal (also referred to as CAAC), i.e., aCAAC-OS layer.

CAAC is a c-axis aligned crystal which has a triangular or hexagonalatomic arrangement when seen from the direction of an a-b plane, asurface, or an interface and in which metal atoms are arranged in alayered manner, or metal atoms and oxygen atoms are arranged in alayered manner along a c-axis, and the direction of the a-axis or theb-axis is varied in the a-b plane (or the surface or the interface),that is, which rotates around the c-axis. A thin film including CAAC iscrystallized along the c-axis but alignment along the a-b planes doesnot necessarily appear.

The CAAC is, in a broad sense, non-single-crystal including a phasewhich has a triangular, hexagonal, regular triangular, or regularhexagonal atomic arrangement when seen from the direction perpendicularto the a-b plane and in which metal atoms are arranged in a layeredmanner or metal atoms and oxygen atoms are arranged in a layered mannerwhen seen from the direction perpendicular to the c-axis direction.

A film including CAAC is not a single crystal, but this does not meanthat the CAAC film is composed of only an amorphous component. Althoughthe CAAC film includes a crystallized portion (crystalline portion), aboundary between one crystalline portion and another crystalline portionis not clear in some cases.

Nitrogen may be substituted for part of oxygen included in the CAAC. Thec-axes of individual crystalline portions included in the CAAC film maybe aligned in one direction (e.g., the direction perpendicular to asurface of a substrate over which the CAAC film is formed, a surface ofthe CAAC film, or an interface of the CAAC film). Alternatively, normalsof the a-b planes of individual crystalline portions included in theCAAC film may be aligned in one direction (e.g., the directionperpendicular to a surface of a substrate over which the CAAC film isformed, a surface of the CAAC film, an interface of the CAAC film, orthe like).

The crystalline oxide semiconductor layer enables a change of electriccharacteristics of the transistor due to irradiation with visible lightor ultraviolet light to be further suppressed, leading to a highlyreliable semiconductor device.

There are three methods for obtaining a crystalline oxide semiconductorlayer having c-axis alignment. The first is a method in which an oxidesemiconductor layer is deposited at a temperature higher than or equalto 200° C. and lower than or equal to 500° C. such that the c-axis issubstantially perpendicular to the top surface. The second is a methodin which an oxide semiconductor layer is deposited thin, and issubjected to heat treatment at a temperature(s) higher than or equal to200° C. and lower than or equal to 700° C., so that the c-axis issubstantially perpendicular to the top surface. The third is a method inwhich a first-layer oxide semiconductor layer is deposited thin, and issubjected to heat treatment at a temperature(s) higher than or equal to200° C. and lower than or equal to 700° C., and a second-layer oxidesemiconductor layer is deposited thereover, so that the c-axis issubstantially perpendicular to the top surface.

The oxide semiconductor layer 403 has a thickness greater than or equalto 1 nm and less than or equal to 30 nm (preferably greater than orequal to 5 nm and less than or equal to 10 nm) and can be formed by asputtering method, a molecular beam epitaxy (MBE) method, a CVD method,a pulse laser deposition method, an atomic layer deposition (ALD)method, or the like as appropriate. The oxide semiconductor layer 403may be formed with a sputtering apparatus which performs deposition inthe state where top surfaces of a plurality of substrates aresubstantially perpendicular to a top surface of a sputtering target.

For example, the CAAC-OS film is formed by a sputtering method with apolycrystalline oxide semiconductor sputtering target. When ions collidewith the sputtering target, a crystal region included in the sputteringtarget may be separated from the target along an a-b plane; in otherwords, a sputtered particle having a plane parallel to an a-b plane(flat-plate-like sputtered particle or pellet-like sputtered particle)may flake off from the sputtering target. In that case, theflat-plate-like sputtered particle reaches a substrate while maintainingtheir crystal state, whereby the CAAC-OS film can be formed.

For the deposition of the CAAC-OS film, the following conditions arepreferably used.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in thedeposition chamber may be reduced. Furthermore, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is −80° C. or lower, preferably −100° C.or lower is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle is likely to occur after the sputteredparticle reaches a substrate surface. Specifically, the substrateheating temperature during the deposition is higher than or equal to100° C. and lower than or equal to 740° C., preferably higher than orequal to 200° C. and lower than or equal to 500° C. By increasing thesubstrate heating temperature during the deposition, when theflat-plate-like sputtered particle reaches the substrate, migrationoccurs on the substrate surface, so that a flat plane of theflat-plate-like sputtered particle is attached to the substrate.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is 30 vol % or higher, preferably 100 vol %.

As an example of the sputtering target, an In—Ga—Zn—O compound target isdescribed below.

The In—Ga—Zn—O compound target, which is polycrystalline, is made bymixing InO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in apredetermined molar ratio, applying pressure, and performing heattreatment at a temperature higher than or equal to 1000° C. and lowerthan or equal to 1500° C. Note that X, Y and Z are given positivenumbers. Here, the predetermined molar ratio of InO_(X) powder toGaO_(Y) powder and ZnO_(Z) powder is, for example, 2:2:1, 8:4:3, 3:1:1,1:1:1, 4:2:3, or 3:1:2. The kinds of powder and the molar ratio formixing powder may be determined as appropriate depending on the desiredsputtering target.

FIGS. 1A to 1E illustrate an example of a method for manufacturing thetransistor 440 a.

First, the oxide insulating layer 436 is formed over the substrate 400having an insulating surface.

There is no particular limitation on a substrate that can be used as thesubstrate 400 having an insulating surface as long as it has heatresistance enough to withstand heat treatment performed later. Forexample, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like, a ceramic substrate, a quartzsubstrate, or a sapphire substrate can be used. A single crystalsemiconductor substrate or a polycrystalline semiconductor substrate ofsilicon, silicon carbide, or the like; a compound semiconductorsubstrate of silicon germanium or the like; an SOI substrate; or thelike can be used as the substrate 400, or the substrate provided with asemiconductor element can be used as the substrate 400.

The semiconductor device may be manufactured using a flexible substrateas the substrate 400. To manufacture a flexible semiconductor device,the transistor 440 a including the oxide semiconductor layer 403 may bedirectly formed over a flexible substrate; or alternatively, thetransistor 440 a including the oxide semiconductor layer 403 may beformed over a substrate, and then may be separated and transferred to aflexible substrate. Note that in order to separate the transistor 440 afrom the manufacturing substrate and transfer it to the flexiblesubstrate, a separation layer may be provided between the manufacturingsubstrate and the transistor 440 a including the oxide semiconductorfilm.

The oxide insulating layer 436 can be formed by a plasma CVD method, asputtering method, or the like using any of silicon oxide, siliconoxynitride, aluminum oxide, aluminum oxynitride, hafnium oxide, galliumoxide, silicon nitride oxide, and aluminum nitride oxide, or a mixedmaterial thereof.

The oxide insulating layer 436 may be either a single layer or a stackedlayer. For example, a silicon oxide film, an In—Hf—Zn-based oxide film,and the oxide semiconductor layer 403 may be stacked in that order overthe substrate 400; a silicon oxide film, an In—Zr—Zn-based oxide filmwith an atomic ratio of In:Zr:Zn=1:1:1, and the oxide semiconductorlayer 403 may be stacked in that order over the substrate 400; or asilicon oxide film, an In—Gd—Zn-based oxide film with an atomic ratio ofIn:Gd:Zn=1:1:1, and the oxide semiconductor layer 403 may be stacked inthat order over the substrate 400.

A silicon oxide film is formed by a sputtering method as the oxideinsulating layer 436 in this embodiment.

Further, a nitride insulating film may be provided between the oxideinsulating layer 436 and the substrate 400. The nitride insulating filmcan be formed using any of silicon nitride, silicon nitride oxide,aluminum nitride, aluminum nitride oxide, or a mixed material of any ofthese, by a plasma CVD method, a sputtering method, or the like.

Next, the oxide semiconductor layer 403 is formed over the oxideinsulating layer 436.

The oxide insulating layer 436, which is in contact with the oxidesemiconductor layer 403, preferably contains oxygen which exceeds atleast the stoichiometric composition ratio in the film (the bulk). Forexample, in the case where a silicon oxide film is used as the oxideinsulating layer 436, the composition formula is SiO_(2+α) (α>0). Byusing such a film as the oxide insulating layer 436, oxygen can besupplied to the oxide semiconductor layer 403, leading to favorablecharacteristics. By a supply of oxygen to the oxide semiconductor layer403, oxygen vacancies in the film can be repaired.

For example, when the oxide insulating layer 436 containing much(excessive) oxygen, which serves as an oxygen supply source, is providedso as to be in contact with the oxide semiconductor layer 403, oxygencan be supplied from the oxide insulating layer 436 to the oxidesemiconductor layer 403. Heat treatment may be performed in the statewhere the oxide semiconductor layer 403 and the oxide insulating layer436 are in contact with each other at least partly to supply oxygen tothe oxide semiconductor layer 403.

In order that hydrogen or water will be not contained in the oxidesemiconductor layer 403 as much as possible in the formation step of theoxide semiconductor layer 403, it is preferable to heat the substrateprovided with the oxide insulating layer 436 in a preheating chamber ina sputtering apparatus as a pretreatment for formation of the oxidesemiconductor layer 403 so that impurities such as hydrogen and moistureadsorbed to the substrate and/or the oxide insulating layer 436 areeliminated and evacuated. As an exhaustion unit provided in thepreheating chamber, a cryopump is preferable.

Therefore, planarizing treatment may be performed on the region of theoxide insulating layer 436 which is in contact with the oxidesemiconductor layer 403. The planarization treatment may be, but notparticularly limited to, polishing treatment (such as chemicalmechanical polishing (CMP)), dry etching treatment, or plasma treatment.

As plasma treatment, reverse sputtering in which an argon gas isintroduced and plasma is generated can be performed. The reversesputtering is a method in which voltage is applied to a substrate sidewith use of an RF power source in an argon atmosphere and plasma isgenerated in the vicinity of the substrate so that a substrate surfaceis modified. Note that instead of an argon atmosphere, a nitrogenatmosphere, a helium atmosphere, an oxygen atmosphere, or the like maybe used. The reverse sputtering can remove particle substances (alsoreferred to as particles or dust) attached to the top surface of theoxide insulating layer 436.

As the planarization treatment, polishing treatment, dry etchingtreatment, or plasma treatment may be performed plural times, or thesetreatments may be performed in combination. In the case where thetreatments are combined, the order of steps is not particularly limitedand may be set as appropriate depending on the roughness of the surfaceof the oxide insulating layer 436.

The oxide semiconductor layer 403 is preferably deposited under acondition such that much oxygen is contained (for example, by asputtering method in an atmosphere where the proportion of oxygen is100%) so as to be a film containing much oxygen (preferably having aregion containing an excess of oxygen as compared to the stoichiometriccomposition ratio of the oxide semiconductor in a crystalline state).

Note that in this embodiment, a target used for forming the oxidesemiconductor layer 403 by a sputtering method is, for example, an oxidetarget having a composition ratio of In:Ga:Zn=3:1:2 [atomic percentage],so that an In—Ga—Zn-based oxide film (IGZO film) is formed.

The relative density (the fill rate) of the metal oxide target is 90% to100% inclusive, preferably 95% to 99.9% inclusive. By using the metaloxide target with high relative density, a dense oxide semiconductorfilm can be formed.

It is preferable to use a high-purity gas from which impurities such ashydrogen, water, a hydroxyl group, or hydride are removed as asputtering gas used when the oxide semiconductor layer 403 is formed.

The substrate is held in a film formation chamber kept under reducedpressure. Then, a sputtering gas from which hydrogen and moisture areremoved is introduced while residual moisture in the film formationchamber is removed, and the oxide semiconductor layer 403 is formed overthe substrate 400 using the above target. In order to remove moistureremaining in the deposition chamber, an entrapment vacuum pump such as acryopump, an ion pump, or a titanium sublimation pump is preferablyused. As an exhaustion unit, a turbo molecular pump to which a cold trapis added may be used. In the deposition chamber which is evacuated withan entrapment vacuum pump such as a cryopump, a hydrogen atom, acompound containing a hydrogen atom such as water (H₂O) (morepreferably, also a compound containing a carbon atom), and the like areremoved, whereby the impurity concentration in the oxide semiconductorlayer 403 formed in the deposition chamber can be reduced.

The oxide insulating layer 436 and the oxide semiconductor layer 403 arepreferably formed in succession without exposure to the air. Accordingto successive formation of the oxide insulating layer 436 and the oxidesemiconductor layer 403 without exposure to the air, impurities such ashydrogen and moisture can be prevented from being adsorbed onto asurface of the oxide insulating layer 436.

The oxide semiconductor layer 403 can be formed by processing an oxidesemiconductor film into an island shape by a photolithography process.

Further, a resist mask for forming the island-shaped oxide semiconductorlayer 403 may be formed by an inkjet method. Formation of the resistmask by an inkjet method needs no photomask; thus, manufacturing costcan be reduced.

Note that the etching of the oxide semiconductor film may be dryetching, wet etching, or both dry etching and wet etching. As an etchantused for wet etching of the oxide semiconductor film, for example, amixed solution of phosphoric acid, acetic acid, and nitric acid, or thelike can be used. In addition, ITO07N (produced by KANTO CHEMICAL CO.,INC.) may also be used.

Further, heat treatment may be performed on the oxide semiconductorlayer 403 in order to remove excess hydrogen (including water and ahydroxyl group) (to perform dehydration or dehydrogenation treatment).The temperature of the heat treatment is higher than or equal to 300° C.and lower than or equal to 700° C., or lower than the strain point ofthe substrate. The heat treatment can be performed under reducedpressure, a nitrogen atmosphere, or the like. For example, the substrateis put in an electric furnace which is a kind of heat treatmentapparatus, and the oxide semiconductor layer 403 is subjected to theheat treatment at 450° C. for an hour in a nitrogen atmosphere.

Further, a heat treatment apparatus used is not limited to an electricfurnace, and a device for heating a process object by heat conduction orheat radiation from a heating element such as a resistance heatingelement may be alternatively used. For example, an RTA (rapid thermalanneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus oran LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the high-temperature gas, an inert gas whichdoes not react with an object to be processed by heat treatment, such asnitrogen or a rare gas like argon, is used.

For example, as the heat treatment, GRTA may be performed as follows.The substrate is put in an inert gas heated at high temperature of 650°C. to 700° C., is heated for several minutes, and is taken out of theinert gas.

Note that in heat treatment, it is preferable that moisture, hydrogen,and the like be not contained in nitrogen or a rare gas such as helium,neon, or argon. The purity of nitrogen or the rare gas such as helium,neon, or argon which is introduced into the heat treatment apparatus isset to preferably 6N (99.9999%) or higher, far preferably 7N (99.99999%)or higher (that is, the impurity concentration is preferably 1 ppm orlower, far preferably 0.1 ppm or lower).

In addition, after the oxide semiconductor layer 403 is heated by theheat treatment, a high-purity oxygen gas, a high-purity dinitrogenmonoxide gas, or ultra dry air (the moisture amount is less than orequal to 20 ppm (−55° C. by conversion into a dew point), preferablyless than or equal to 1 ppm, further preferably less than or equal to 10ppb, in the measurement with the use of a dew point meter of a cavityring down laser spectroscopy (CRDS) system) may be introduced into thesame furnace. It is preferable that water, hydrogen, or the like be notcontained in the oxygen gas or the dinitrogen monoxide gas. The purityof the oxygen gas or the dinitrogen monoxide gas which is introducedinto the heat treatment apparatus is preferably 6N or more, farpreferably 7N or more (i.e., the impurity concentration in the oxygengas or the dinitrogen monoxide gas is preferably 1 ppm or lower, farpreferably 0.1 ppm or lower). The oxygen gas or the dinitrogen monoxidegas acts to supply oxygen that is a main component of the oxidesemiconductor and that is reduced by the step for removing an impurityfor the dehydration or dehydrogenation, so that the oxide semiconductorlayer 403 can be a high-purified, i-type (intrinsic) oxide semiconductorfilm.

Note that the heat treatment for dehydration or dehydrogenation can beperformed in the process of manufacturing the transistor 440 a anytimeafter formation of the oxide semiconductor film which has not beenprocessed into the oxide semiconductor layer 403 and before formation ofthe insulating film 407. For example, the heat treatment may beperformed after formation of the oxide semiconductor film or afterformation of the island-shaped oxide semiconductor layer 403.

Further, the heat treatment for dehydration or dehydrogenation may beperformed more than once or may be combined with another heat treatment.

When the heat treatment for dehydration or dehydrogenation is performedin the state where the oxide insulating layer 436 is covered with theoxide semiconductor film which has not been processed into theisland-shaped oxide semiconductor layer 403, oxygen contained in theoxide insulating layer 436 can be prevented from being released by theheat treatment, which is preferable.

Further or alternatively, oxygen (which includes at least one of anoxygen radical, an oxygen atom, and an oxygen ion) may be added to theoxide semiconductor layer after being subjected to the dehydration ordehydrogenation treatment to supply oxygen to the oxide semiconductorlayer.

Oxygen which is added to the dehydrated or dehydrogenated oxidesemiconductor layer 403 to supply oxygen to the film can highly purifythe oxide semiconductor layer 403 and make the film an i-type(intrinsic). Variation in electric characteristics of a transistorhaving a highly-purified and i-type (intrinsic) oxide semiconductorlayer 403 is suppressed, and the transistor is electrically stable.

As a method for introducing oxygen, an ion implantation method, an iondoping method, a plasma immersion ion implantation method, plasmatreatment, or the like may be used.

In the step of addition of oxygen, oxygen may be directly added to theoxide semiconductor layer 403 or added to the oxide semiconductor layer403 through another film such as the gate insulating film 402 or theinsulating film 407. An ion implantation method, an ion doping method, aplasma immersion ion implantation method, or the like may be employedfor the addition of oxygen through another film, whereas plasmatreatment or the like can also be employed for the addition of oxygendirectly into an exposed oxide semiconductor layer 403.

As described above, the addition of oxygen into the oxide semiconductorlayer 403 can be performed anytime after dehydration or dehydrogenationtreatment is performed thereon. Further, oxygen may be added pluraltimes into the dehydrated or dehydrogenated oxide semiconductor layer403.

Next, a conductive film to be a source electrode layer and a drainelectrode layer (including a wiring formed in the same layer as thesource electrode layer and the drain electrode layer) is formed over theoxide semiconductor layer 403. The conductive film is formed using amaterial that can withstand heat treatment in a later step. As aconductive film used for the source electrode layer and the drainelectrode layer, for example, a metal film containing an elementselected from Al, Cr, Cu, Ta, Ti, Mo, and W and a metal nitride filmcontaining any of the above elements as its main component (a titaniumnitride film, a molybdenum nitride film, and a tungsten nitride film)can be used. A metal film having a high melting point such as Ti, Mo, W,or the like or a metal nitride film of any of these elements (a titaniumnitride film, a molybdenum nitride film, and a tungsten nitride film)may be stacked on one of or both of a lower side or an upper side of ametal film of Al, Cu, or the like. Alternatively, the conductive filmused for the source electrode layer and the drain electrode layer may beformed using a conductive metal oxide. As the conductive metal oxide,indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), indiumoxide-tin oxide (In₂O₃—SnO₂), indium oxide-zinc oxide (In₂O₃—ZnO), orany of these metal oxide materials in which silicon oxide is containedcan be used.

Through a photolithography process, a resist mask is formed over theconductive film, and selective etching is performed thereon, so that thesource electrode layer 405 a and the drain electrode layer 405 b areformed, and then, the resist mask is removed.

Next, the gate insulating film 402 covering the oxide semiconductorlayer 403, the source electrode layer 405 a, and the drain electrodelayer 405 b is formed (see FIG. 1A).

To improve the coverage with the gate insulating film 402, theabove-described planarizing treatment may be performed also on the topsurface of the oxide semiconductor layer 403, and top surfaces of thesource electrode layer 405 a and the drain electrode layer 405 b. It ispreferable that the flatness of the top surface of the oxidesemiconductor layer 403 and the top surfaces of the source electrodelayer 405 a and the drain electrode layer 405 b be good particularlywhen the thickness of the gate insulating film 402 is small.

The gate insulating film 402 can be formed to have a thickness greaterthan or equal to 1 nm and less than or equal to 20 nm by a sputteringmethod, an MBE method, a CVD method, a pulse laser deposition method, anALD method, or the like as appropriate. Alternatively, the gateinsulating film 402 may be formed with a sputtering apparatus where filmformation is performed with surfaces of a plurality of substrates setsubstantially perpendicular to a surface of a sputtering target.

The gate insulating film 402 can be formed using a silicon oxide film, agallium oxide film, an aluminum oxide film, a silicon nitride film, asilicon oxynitride film, an aluminum oxynitride film, or a siliconnitride oxide film. It is preferable that the gate insulating film 402include oxygen in a portion which is in contact with the oxidesemiconductor layer 403. In particular, the gate insulating film 402preferably contains a large amount of oxygen which exceeds at least thestoichiometric ratio in (a bulk of) the film. For example, in the casewhere a silicon oxide film is used as the gate insulating film 402, thecomposition formula is SiO_(2+α) (α>0). In this embodiment, a siliconoxide film of SiO_(2+α) (α>0) is used as the gate insulating film 402.By using the silicon oxide film as the gate insulating film 402, oxygencan be supplied to the oxide semiconductor layer 403, leading to goodcharacteristics. Further, the gate insulating film 402 is preferablyformed in consideration of the size of a transistor to be formed and thestep coverage with the gate insulating film 402.

When the gate insulating film 402 is formed using a high-k material suchas hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0,y>0)), hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)) to which nitrogen isadded, hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)), or lanthanum oxide,gate leakage current can be reduced. Further, the gate insulating film402 may have either a single-layer structure or a stacked-layerstructure.

Then, the gate electrode layer 401 is formed over the gate insulatingfilm 402 by a plasma CVD method, a sputtering method, or the like (seeFIG. 1B). The gate electrode layer 401 can be formed using a metalmaterial such as molybdenum, titanium, tantalum, tungsten, aluminum,copper, chromium, neodymium, or scandium or an alloy material whichcontains any of these materials as its main component. Alternatively, asemiconductor film typified by a polycrystalline silicon film doped withan impurity element such as phosphorus, or a silicide film such as anickel silicide film may be used as the gate electrode layer 401. Thegate electrode layer 401 may have a single-layer structure or astacked-layer structure.

The gate electrode layer 401 can also be formed using a conductivematerial such as indium tin oxide, indium oxide containing tungstenoxide, indium zinc oxide containing tungsten oxide, indium oxidecontaining titanium oxide, indium tin oxide containing titanium oxide,indium zinc oxide, or indium tin oxide to which silicon oxide is added.It is also possible that the gate electrode layer 401 has a stackedstructure of the above conductive material and the above metal material.

As one layer of the gate electrode layer 401 which is in contact withthe gate insulating film 402, a metal oxide containing nitrogen,specifically, an In—Ga—Zn—O film containing nitrogen, an In—Sn—O filmcontaining nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—Ofilm containing nitrogen, a Sn—O film containing nitrogen, an In—O filmcontaining nitrogen, or a metal nitride (e.g., InN or SnN) film can beused. These films each have a work function higher than or equal to 5eV, preferably higher than or equal to 5.5 eV; thus, when these are usedas the gate electrode layer, the threshold voltage of the electriccharacteristics of the transistor can be positive. Accordingly, aso-called normally-off switching element can be provided.

Next, a dopant 421 is introduced into the oxide semiconductor layer 403with the use of the gate electrode layer 401, the source electrode layer405 a, and the drain electrode layer 405 b as masks, whereby thelow-resistance regions 404 a and 404 b are formed.

The dopant 421 is not added to the oxide semiconductor layer 403 in theregions under the source electrode layer 405 a and the drain electrodelayer 405 b in some cases, or the dopant 421 is added to the oxidesemiconductor layer 403 in the regions under the source electrode layer405 a and the drain electrode layer 405 b such that the dopantconcentration in each region is lower and the resistance in each regionis higher than that of the other region in some cases, depending on thethickness of each of the source electrode layer 405 a and the drainelectrode layer 405 b and the condition of addition of the dopant 421.

In a transistor 440 c in FIG. 2B, a tungsten film with small thickness,for example 10 nm, is formed as the source electrode layer 405 a and thedrain electrode layer 405 b. Owing to the above-described smallthickness of each of the source electrode layer 405 a and the drainelectrode layer 405 b, when a dopant is introduced into the oxidesemiconductor layer 403 to form low-resistance regions, the dopant canalso be introduced into the oxide semiconductor layer 403 which is belowthe source electrode layer 405 a and the drain electrode layer 405 b,through the source electrode layer 405 a and the drain electrode layer405 b. As a result, in the transistor 440 c, the low-resistance regions404 a and 404 b are formed in the oxide semiconductor layer 403 which isbelow the source electrode layer 405 a and the drain electrode layer 405b.

The dopant 421 is an impurity by which the electrical conductivity ofthe oxide semiconductor layer 403 is changed. One or more selected fromthe following can be used as the dopant 421: Group 15 elements (typicalexamples thereof are phosphorus (P), arsenic (As), and antimony (Sb)),boron (B), aluminum (Al), nitrogen (N), argon (Ar), helium (He), neon(Ne), indium (In), fluorine (F), chlorine (Cl), titanium (Ti), and zinc(Zn).

The dopant 421 can be introduced into the oxide semiconductor layer 403through other films (e.g., the insulating film 407, the source electrodelayer 405 a, and the drain electrode layer 405 b) by an implantationmethod. As the method for introducing the dopant 421, an ionimplantation method, an ion doping method, a plasma immersion ionimplantation method, or the like can be used. In the case where theabove method is used, it is preferable to use a single ion of the dopant421, a fluoride ion, or a chloride ion.

The introduction of the dopant 421 may be controlled by setting theaddition conditions such as the accelerated voltage and the dosage, orthe thickness of the films through which the dopant passes asappropriate. In this embodiment, boron is used as the dopant 421, whoseion is introduced by an ion implantation method. The dosage ispreferably set to be greater than or equal to 1×10¹³ ions/cm² and lessthan or equal to 5×10¹⁶ ions/cm².

The concentration of the dopant 421 in the low-resistance region ispreferably greater than or equal to 5×10¹⁸/cm³ and less than or equal to1×10²²/cm³.

The dopant 421 may be introduced while the substrate 400 is heated.

The introduction of the dopant 421 into the oxide semiconductor layer403 may be performed plural times, and the number of kinds of dopant maybe plural.

Further, heat treatment may be performed thereon after the introductionof the dopant 421. The heat treatment is preferably performed at atemperature(s) higher than or equal to 300° C. and lower than or equalto 700° C. (further preferably higher than or equal to 300° C. and lowerthan or equal to 450° C.) for one hour under an oxygen atmosphere. Theheat treatment may be performed under a nitrogen atmosphere, reducedpressure, or the air (ultra-dry air).

In the case where the oxide semiconductor layer 403 is a crystallineoxide semiconductor film, the oxide semiconductor layer 403 may bepartly amorphized by the introduction of the dopant 421. In that case,the crystallinity of the oxide semiconductor layer 403 can be recoveredby performing a heat treatment thereon after the introduction of thedopant 421.

Thus, the oxide semiconductor layer 403 in which the low-resistanceregions 404 a and 404 b are formed with the channel formation region 409sandwiched between the low-resistance regions 404 a and 404 b.

Through the above-described process, the transistor 440 a of thisembodiment can be manufactured (see FIG. 1C). With the oxidesemiconductor layer 403 which contains at least four kinds of elementsof indium, gallium, zinc, and oxygen, and has a composition ratio(atomic percentage) of indium as twice or more as a composition ratio ofgallium and a composition ratio of zinc, the transistor 440 a can haveexcellent on-state characteristics (field-effect mobility), smalloff-state current, and high reliability.

Next, the insulating film 407 is formed over the oxide semiconductorlayer 403, the source electrode layer 405 a, the drain electrode layer405 b, the gate insulating film 402, and the gate electrode layer 401(see FIG. 1D).

The insulating film 407 including the metal element can be formed by aplasma-enhanced CVD method, a sputtering method, an evaporation method,or the like. As the insulating film 407, an inorganic insulating filmsuch as a silicon oxide film, a silicon oxynitride film, an aluminumoxide film, an aluminum oxynitride film, or a gallium oxide film can betypically used.

Alternatively, as the insulating film 407, an aluminum oxide film, ahafnium oxide film, a magnesium oxide film, a zirconium oxide film, alanthanum oxide film, a barium oxide film, or a metal nitride film(e.g., an aluminum nitride film) can be used.

The insulating film 407 can be either a single film or a stacked film.The insulating film 407 can be a stack of a silicon oxide film and analuminum oxide film, for example.

The insulating film 407 is preferably formed by a method such as asputtering method, in which an impurity such as water or hydrogen doesnot enter the insulating film 407. In addition, it is preferable thatthe insulating film 407 include an excess of oxygen on the side closerto the oxide semiconductor layer 403 because the excess of oxygen servesas a supply source of oxygen for the oxide semiconductor layer 403.

In this embodiment, a silicon oxide film with a thickness of 100 nm isformed as the insulating film 407 by a sputtering method. The siliconoxide film can be formed by a sputtering method under a rare gas(typically, argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere containing a rare gas and oxygen.

In order to remove residual moisture from the deposition chamber of theinsulating film 407 in a manner similar to that of the deposition of theoxide semiconductor film, an entrapment vacuum pump (such as a cryopump)is preferably used. When the insulating film 407 is deposited in thedeposition chamber evacuated using a cryopump, the impurityconcentration of the insulating film 407 can be reduced. As anevacuation unit for removing moisture remaining in the depositionchamber of the insulating film 407, a turbo molecular pump provided witha cold trap may be used.

It is preferable that a high-purity gas in which an impurity such ashydrogen, water, a hydroxyl group, or hydride is reduced be used as thesputtering gas for the formation of the insulating film 407.

The aluminum oxide film which can be used as the insulating film 407provided over the oxide semiconductor layer 403 has a high blockingeffect by which both of oxygen and impurities such as hydrogen ormoisture is prevented from being passed through the film.

Therefore, in and after the manufacturing process, the aluminum oxidefilm functions as a protective film for preventing entry of an impuritysuch as hydrogen or moisture, which causes a change, into the oxidesemiconductor layer 403 and release of oxygen, which is a mainconstituent material of the oxide semiconductor, from the oxidesemiconductor layer 403.

Further, a planarization insulating film may be formed thereover inorder to reduce surface roughness due to the transistor. As theplanarization insulating film, an organic material such as apolyimide-based resin, an acrylic-based resin, or abenzocyclobutene-based resin can be used. Other than such organicmaterials, it is also possible to use a low-dielectric constant material(a low-k material) or the like. Note that the planarization insulatingfilm may be formed by stacking a plurality of insulating films formedfrom these materials.

Further, respective openings reaching the source electrode layer 405 aand the drain electrode layer 405 b are formed in the gate insulatingfilm 402 and the insulating film 407, and a wiring layer 465 a and awiring layer 465 b electrically connected to the source electrode layer405 a and the drain electrode layer 405 b, respectively, are formed inthe openings (see FIG. 1E). With the use of this wiring layers 465 a and465 b, the transistor is connected to another transistor, which can leadto formation of a variety of circuits.

Alternatively, as a transistor 440 d in FIG. 2C, the wirings 465 a and465 b may be formed directly on the oxide semiconductor layer 403without providing the source electrode layer 405 a and the drainelectrode layer 405 b.

The wiring layers 465 a and 465 b can be formed with a material and amethod which are similar to those of the gate electrode layer 401, thesource electrode layer 405 a, and the drain electrode layer 405 b. Forexample, as the wiring layers 465 a and 465 b, a stack of a tantalumnitride film and a copper film or a stack of a tantalum nitride film anda tungsten film can be used.

In the oxide semiconductor layer 403 which is highly purified and whoseoxygen vacancies are repaired, impurities such as hydrogen and water aresufficiently removed; the hydrogen concentration in the oxidesemiconductor layer 403 is less than or equal to 5×10¹⁹/cm³, preferablyless than or equal to 5×10¹⁸/cm³. The hydrogen concentration in theoxide semiconductor layer 403 is measured by secondary ion massspectrometry (SIMS).

The current value in the off state (off-state current value) of thetransistor 440 a using the highly purified oxide semiconductor layer 403containing an excess of oxygen that repairs an oxygen vacancy accordingto this embodiment is less than or equal to 100 zA per micrometer ofchannel width at room temperature (1 zA (zeptoampere)=1×10⁻²¹A),preferably less than or equal to 50 zA/mm.

In this manner, a structure of a transistor including an oxidesemiconductor in a channel formation region in which the thresholdvoltage of electric characteristics of the transistor can be positive,which is a so-called normally-off switching element, and a manufacturingmethod thereof can be provided.

Further, in order to achieve a semiconductor device having higherperformance, a structure for improving on-state characteristics of thetransistor (e.g., on-state current and field-effect mobility) and forachieving high-speed response and high-speed operation of thesemiconductor device and a manufacturing method thereof can be provided.

In addition, a highly reliable semiconductor device in which a thresholdvoltage does not easily shift even in a long-time use can be provided.

Embodiment 2

In this embodiment, another embodiment of a semiconductor device and amethod for manufacturing the semiconductor device are described withreference to FIGS. 3A to 3C and FIGS. 4A to 4E. The same portion as or aportion having a function similar to those in the above embodiment canbe formed in a manner similar to that described in the above embodiment,and also the steps similar to those in the above embodiment can beperformed in a manner similar to that described in the above embodiment,and repetitive description is omitted. In addition, detailed descriptionof the same portions is not repeated.

A transistor 450 illustrated in FIGS. 3A to 3C is an example of atop-gate transistor. FIG. 3A is a top view, FIG. 3B is a cross-sectionalview taken along two-dot chain line X-Y in FIG. 3A, and FIG. 3C is across-sectional view taken along two-dot chain line V-W in FIG. 3A.

As shown in FIG. 3B which is a cross-sectional view in the channellength direction, the transistor 450 includes, over the substrate 400having an insulating surface over which the oxide insulating layer 436is formed, first oxide semiconductor layers 408 a and 408 b, a secondoxide semiconductor layer 403 including the channel formation region 409and low-resistance regions 414 a and 414 b, the source electrode layer405 a, the drain electrode layer 405 b, the gate insulating film 402,and the gate electrode layer 401. The oxide semiconductor layers 408 aand 408 b are separated from each other on and in contact with the oxideinsulating layer 436. The oxide semiconductor layer 403 is in contactwith the oxide semiconductor layers 408 a and 408 b and the oxideinsulating layer 436.

FIG. 3C is a cross-sectional view in the channel width direction. Theoxide semiconductor layer 403 has a taper angle of 20° to 50°. When theoxide semiconductor layer 403 has a perpendicular end portion, oxygen islikely to be eliminated, and thus, oxygen defects easily occur. Incontrast, when the oxide semiconductor layer 403 is tapered, theoccurrence of oxygen defects is suppressed and thus the occurrence ofleakage current (parasitic channel) in the transistor 450 is reduced.

The oxide semiconductor layers 408 a and 408 b are provided below theoxide semiconductor layer 403 with a thickness of 3 nm to 5 nm, so thatcontact resistance between the oxide semiconductor layer 403 and thesource and drain electrode layers 405 a and 405 b can be reduced.

The low-resistance regions 414 a and 414 b can be formed by introducinga dopant into the oxide semiconductor layer 403 with the use of the gateelectrode layer 401 as a mask. Further or alternatively, thelow-resistance regions can be formed by diffusing a metal element. Whenthe low-resistance regions are formed by introducing a dopant anddiffusing a metal element, contact resistance with the wiring layers canbe further reduced.

Further, a sidewall insulating layer may be formed on a side surface ofthe gate electrode layer 401. The transistor 450 includes thin sidewallinsulating layers 412 a and 412 b on the side surfaces of the gateelectrode layer 401. The sidewall insulating layers 412 a and 412 b maybe formed on the side surface of the gate electrode layer 401 in aself-aligned manner by forming an insulating film to cover the gateelectrode layer 401 and then processing the insulating film byanisotropic etching by a reactive ion etching (RIE) method. There is noparticular limitation on the insulating film; for example, a siliconoxide film with favorable step coverage, which is formed by reactingtetraethyl orthosilicate (TEOS), silane, or the like with oxygen,nitrous oxide, or the like, can be used. The insulating film can beformed by a thermal CVD method, a plasma CVD method, an atmosphericpressure CVD method, a bias ECRCVD method, a sputtering method, or thelike. A silicon oxide film formed by a low temperature oxidation (LTO)method may also be used.

With the sidewall insulating layers 412 a and 412 b, the gate electrodelayer 401 and the low-resistance regions 414 a and 414 b can beprevented from being short-circuited.

When a dopant is introduced into the entire oxide semiconductor layers408 a and 408 b to form the low-resistance regions, the transistor canbe electrically connected with another conductive layer, below the oxidesemiconductor layers 408 a and 408 b, i.e., from the oxide insulatinglayer 436 side.

When the oxide semiconductor layer 403 is formed using an IGZO filmwhich contains at least four kinds of elements of indium, gallium, zinc,and oxygen, and has a composition ratio (atomic percentage) of indium astwice or more as a composition ratio of gallium and a composition ratioof zinc has high field-effect mobility, the thickness of the oxidesemiconductor layer 403 is decreased to 3 nm to 5 nm, whereby atransistor can be prevented from being normally-on caused by ashort-channel effect.

For the oxide semiconductor layers 408 a and 408 b, an indium oxide, atin oxide, a zinc oxide; a two-component metal oxide such as anIn—Zn—based oxide, a Sn—Zn-based oxide, an Al—Zn—based oxide, aZn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or anIn—Ga-based oxide; a three-component metal oxide such as anIn—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-basedoxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, anAl—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide,an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-basedoxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, anIn—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide,an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-basedoxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or anIn—Lu—Zn-based oxide; or a four-component metal oxide such as anIn—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide can be used.

As the oxide semiconductor layers 408 a and 408 b, an oxidesemiconductor layer having a high conductivity can be used.

In this embodiment, an oxide semiconductor layer formed using an oxidetarget having a composition ratio of indium:gallium:zinc=1:1:1 is usedas each of the oxide semiconductor layers 408 a and 408 b.

The thickness of each of the oxide semiconductor layers 408 a and 408 bis preferably 20 nm to 50 nm.

An example of a method for manufacturing the transistor 450 is describedwith reference to FIGS. 4A to 4E.

First, the oxide insulating layer 436 is formed over the substrate 400having an insulating surface and the oxide semiconductor film 444 isformed over the oxide insulating layer 436 (see FIG. 4A). In thisembodiment, the oxide semiconductor film 444 is formed using an oxidetarget having a composition ratio of indium:gallium:zinc=1:1:1, by asputtering method.

Then, the oxide semiconductor film 444 is processed into an island shapeby a photolithography process to form the pair of oxide semiconductorlayers 408 a and 408 b which are separated from each other. The oxidesemiconductor layer 403 is formed in contact with the oxidesemiconductor layers 408 a and 408 b and the oxide insulating layer 436(see FIG. 4B). The oxide semiconductor layer 403 is formed using anoxide target having a composition ratio of indium:gallium:zinc=3:1:2 bya sputtering method. The oxide semiconductor layer 403 is preferablytapered, and in this embodiment, has a taper angle of 30°.

Next, over the oxide semiconductor layer 403, the gate insulating film402, the gate electrode layer 401, the sidewall insulating layers 412 aand 412 b which cover side surfaces of the gate electrode layer 401 areformed (see FIG. 4C). The gate insulating film 402 can be formed byforming an insulating film over the oxide semiconductor layer 403 andetching the insulating film with the use of the gate electrode layer 401and the sidewall insulating layers 412 a and 412 b as masks. Note thatpart of the oxide semiconductor layer 403 is exposed.

Next, a film 417 including a metal element is formed over the oxidesemiconductor layer 403, the gate insulating film 402, and the gateelectrode layer 401 to be in contact with the part of the oxidesemiconductor layer 403.

As the film 417 including the metal element, a metal film, a metal oxidefilm, a metal nitride film, and the like are used.

As the metal element included in the film including the metal element,one or more selected from aluminum (Al), titanium (Ti), molybdenum (Mo),tungsten (W), hafnium (Hf), tantalum (Ta), lanthanum (La), barium (Ba),magnesium (Mg), zirconium (Zr), and nickel (Ni) can be used. As the filmincluding the metal element, a metal film, a metal oxide film, or ametal nitride film including at least one of the above-described metalelements (such a metal nitride film is, for example, a titanium nitridefilm, a molybdenum nitride film, or a tungsten nitride film) can beused. Further, a dopant such as phosphorus (P) or boron (B) may beincluded in the film including the metal element. In this embodiment,the film 417 including the metal element has electrical conductivity.

The film 417 including the metal element can be formed by aplasma-enhanced CVD method, a sputtering method, an evaporation method,or the like. The thickness of the film 417 including the metal elementmay be greater than or equal to 5 nm and less than or equal to 30 nm.

In this embodiment, a 10-nm-thick aluminum film is formed by asputtering method as the film 417 including the metal element.

Next, the dopant 421 is selectively introduced into the oxidesemiconductor layer 403 through the film 417 including the metal elementwith the gate insulating film 402, the gate electrode layer 401, and thesidewall insulating layers 412 a and 412 b as a mask, so thatlow-resistance regions are formed (see FIG. 4D).

The dopant 421 is an impurity by which the electrical conductivity ofthe oxide semiconductor layer 403 is changed. One or more selected fromthe following can be used as the dopant 421: Group 15 elements (typicalexamples thereof are phosphorus (P), arsenic (As), and antimony (Sb)),boron (B), aluminum (Al), nitrogen (N), argon (Ar), helium (He), neon(Ne), indium (In), fluorine (F), chlorine (Cl), titanium (Ti), and zinc(Zn).

The dopant may be included in the film 417 including the metal element.

The dopant 421 is introduced into the oxide semiconductor layer 403through the film 417 including the metal element by an implantationmethod. As the method for introducing the dopant 421, an ionimplantation method, an ion doping method, a plasma immersion ionimplantation method, or the like can be used. In the case where theabove method is used, it is preferable to use a single ion of the dopant421 or a hydride ion, a fluoride ion, or a chloride ion.

The introduction of the dopant 421 may be controlled by setting theaddition conditions such as the accelerated voltage and the dosage, orthe thickness of the film 417 including the metal element asappropriate. The dose can be greater than or equal to 1×10¹³ ions/cm²and less than or equal to 5×10¹⁶ ions/cm². For example, for introductionof an boron ion by an ion implantation method using boron, theaccelerated voltage and the dosage may be set to 15 kV and 1×10¹⁵ions/cm², respectively.

The concentration of the dopant 421 in the low-resistance region ispreferably greater than or equal to 5×10¹⁸/cm³ and less than or equal to1×10²²/cm³.

The substrate 400 may be heated in introducing the dopant.

The introduction of the dopant 421 into the oxide semiconductor layer403 may be performed more than once, and the number of kinds of dopantmay be plural.

Further, heat treatment may be performed thereon after the introductionof the dopant 421. The heat treatment is preferably performed at atemperature(s) higher than or equal to 300° C. and lower than or equalto 700° C. (further preferably higher than or equal to 300° C. and lowerthan or equal to 450° C.) for one hour in an oxygen atmosphere. The heattreatment may be performed in a nitrogen atmosphere, reduced pressure,or the air (ultra-dry air).

Next, heat treatment is performed in the state where the film 417including the metal element and the oxide semiconductor layer 403 arepartly in contact with each other. The heat treatment is preferablyperformed in an oxygen atmosphere. The heat treatment can also beperformed under reduced pressure or a nitrogen atmosphere. The heatingtemperature may be set to be higher than or equal to 100° C. and lowerthan or equal to 700° C., preferably higher than or equal to 200° C. andlower than or equal to 400° C.

For example, the substrate is put in an electric furnace which is one ofheat treatment apparatuses, and heat treatment is performed on the film417 including the metal element and the oxide semiconductor layer 403 at200° C. for one hour in an oxygen atmosphere.

Further, a heat treatment apparatus used is not limited to an electricfurnace, and a device for heating a process object by heat conduction orheat radiation from a heating element such as a resistance heatingelement may be alternatively used. For example, an RTA (rapid thermalanneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus oran LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the high-temperature gas, an inert gas whichdoes not react with an object to be processed by heat treatment, such asnitrogen or a rare gas like argon, is used.

For example, as the heat treatment, GRTA may be performed as follows.The substrate is put in an inert gas heated at high temperature of 650°C. to 700° C., is heated for several minutes, and is taken out of theinert gas.

The heat treatment may be performed in an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is 20 ppm or lower,preferably 1 ppm or lower, more preferably 10 ppb or lower), or a raregas (argon, helium, or the like). Note that it is preferable that water,hydrogen, or the like be not contained in the atmosphere of nitrogen,oxygen, ultra-dry air, or a rare gas. It is also preferable that thepurity of nitrogen, oxygen, or the rare gas which is introduced into aheat treatment apparatus be set to be 6N (99.9999%) or higher,preferably 7N (99.99999%) or higher (that is, the impurity concentrationis 1 ppm or lower, preferably 0.1 ppm or lower).

By the heat treatment, the metal element is introduced into the oxidesemiconductor layer 403 from the film 417 including the metal element,so that low-resistance regions 414 a and 414 b are formed. In thismanner, in the oxide semiconductor layer 403, the low-resistance regions414 a and 414 b including the dopant and the metal element are formedbetween which a channel formation region 409 is provided.

In this embodiment, boron is used as the dopant and aluminum is used asthe metal element, and therefore the low-resistance regions 414 a and414 b contain boron and aluminum.

Then, the film 417 including the metal element is removed by etching.The film 417 including the metal element is removed by wet etching, inthis embodiment.

Through the above process, the transistor 450 of this embodiment can bemanufactured. With the oxide semiconductor layer 403 including thelow-resistance regions 414 a and 414 b with the channel formation region409 provided therebetween in the channel length direction, on-statecharacteristics (e.g., on-state current and field-effect mobility) ofthe transistor 450 are increased, which enables high-speed operation andhigh-speed response of the transistor 450.

The low-resistance regions 414 a and 414 b each can be functioned as asource region or a drain region. With the low-resistance regions 414 aand 414 b, the electrical field applied to the channel formation region409 formed between the low-resistance regions 414 a and 414 b can berelaxed. Further, electrical connection between the oxide semiconductorlayer 403 and each of the source electrode layer 405 a and the drainelectrode layer 405 b in the low-resistance regions 414 a and 414 b,respectively, can reduce the contact resistance between the oxidesemiconductor layer 403 and each of the source electrode layer 405 a andthe drain electrode layer 405 b.

Further, a planarization insulating film may be formed thereover inorder to reduce surface roughness due to the transistor. As theplanarization insulating film, an organic material such as apolyimide-based resin, an acrylic-based resin, or abenzocyclobutene-based resin can be used. Other than such organicmaterials, it is also possible to use a low-dielectric constant material(a low-k material) or the like. Note that the planarization insulatingfilm may be formed by stacking a plurality of insulating films formedfrom these materials.

In this embodiment, a planarization insulating film 415 is formed overthe transistor 450. Further, openings reaching the oxide semiconductorlayer 403 are formed in the planarization insulating film 415, and thesource electrode layer 405 a and the drain electrode layer 405 b areformed so as to be electrically connected to the oxide semiconductorlayer 403 through the openings (see FIG. 4E).

In this manner, a structure of a transistor including an oxidesemiconductor in a channel formation region in which the thresholdvoltage of electric characteristics of the transistor can be positive,which is a so-called normally-off switching element, and a manufacturingmethod thereof can be provided.

Further, in order to achieve a semiconductor device having higherperformance, a structure for improving on-state characteristics of thetransistor (e.g., on-state current and field-effect mobility) and forachieving high-speed response and high-speed operation of thesemiconductor device and a manufacturing method thereof can be provided.

In addition, a semiconductor device in which reliability is high andthreshold voltage does not easily shift even in a long-time use can beprovided.

This embodiment can be implemented combining with the other embodimentsas appropriate.

Embodiment 3

In this embodiment, an example of a semiconductor device which includesthe transistor described in Embodiment 1 or 2, which can hold storeddata even when not powered, and which does not have a limitation on thenumber of write cycles, will be described with reference to drawings.Note that a transistor 162 included in the semiconductor device in thisembodiment is the transistor described in Embodiment 1 or 2. Any of thetransistors described in Embodiment 1 or 2 can be used as the transistor162.

Since the off-state current of the transistor 162 is small, stored datacan be held for a long time owing to such a transistor. In other words,power consumption can be sufficiently reduced because a semiconductorstorage device in which refresh operation is unnecessary or thefrequency of refresh operation is extremely low can be provided.

FIGS. 5A and 5B illustrate an example of a structure of a semiconductordevice. FIG. 5A is a cross-sectional view of the semiconductor device,FIG. 5B is a plan view of the semiconductor device, and FIG. 5C is acircuit diagram of the semiconductor device. Here, FIG. 5A correspondsto a cross section along line C1-C2 and line D1-D2 in FIG. 5B.

The semiconductor device illustrated in FIGS. 5A and 5B includes atransistor 160 including a first semiconductor material in a lowerportion, and a transistor 162 including a second semiconductor materialin an upper portion. The transistor 162 can have the same structure asthat described in Embodiment 1 or 2.

Here, the first semiconductor material and the second semiconductormaterial are preferably materials having different band gaps. Forexample, the first semiconductor material may be a semiconductormaterial other than an oxide semiconductor (e.g., silicon) and thesecond semiconductor material may be an oxide semiconductor. Atransistor including a material other than an oxide semiconductor canoperate at high speed easily. On the other hand, a transistor includingan oxide semiconductor can hold charge for a long time owing to itscharacteristics.

Although all the transistors are n-channel transistors here, p-channeltransistors can be used. The technical nature of the disclosed inventionis to use an oxide semiconductor in the transistor 162 so that data canbe held. Therefore, it is not necessary to limit a specific structure ofthe semiconductor device, such as a material of the semiconductor deviceor a structure of the semiconductor device, to the structure describedhere.

The transistor 160 in FIG. 5A includes a channel formation region 116provided over a substrate 185 including a semiconductor material (e.g.,silicon), impurity regions 120 with the channel formation region 116provided therebetween, metal compound regions 124 in contact with theimpurity regions 120, a gate insulating layer 108 provided over thechannel formation region 116, and a gate electrode 110 provided over thegate insulating layer 108. Note that a transistor whose source electrodeand drain electrode are not illustrated in a drawing may be referred toas a transistor for the sake of convenience. Further, in such a case, indescription of a connection of a transistor, a source region and asource electrode are collectively referred to as a “source electrode,”and a drain region and a drain electrode are collectively referred to asa “drain electrode”. That is, in this specification, the term “sourceelectrode” may include a source region.

An element isolation insulating layer 106 is provided over the substrate185 to surround the transistor 160. An insulating layer 130 is providedto cover the transistor 160. Note that in order to realize highintegration, it is preferable that, as in FIG. 5A, the transistor 160does not have a sidewall insulating layer. On the other hand, when thecharacteristics of the transistor 160 have priority, the sidewallinsulating layer may be formed on a side surface of the gate electrode110 and the impurity regions 120 may include a region having a differentimpurity concentration.

The transistor 162 shown in FIG. 5A includes an oxide semiconductor inthe channel formation region. Here, an oxide semiconductor layer 144included in the transistor 162 is preferably highly purified. By using ahighly purified oxide semiconductor, the transistor 162 can haveextremely favorable off-state current characteristics.

An insulating layer 150 having a single-layer structure or astacked-layer structure is provided over the transistor 162. Inaddition, a conductive layer 148 b is provided in a region overlappingwith the electrode layer 142 a of the transistor 162 with the insulatinglayer 150 provided therebetween, and the electrode layer 142 a, theinsulating layer 150, and the conductive layer 148 b form a capacitor164. That is, the electrode layer 142 a of the transistor 162 functionsas one electrode of the capacitor 164, and the conductive layer 148 bfunctions as the other electrode of the capacitor 164. Note that thecapacitor 164 may be omitted if a capacitor is not needed.Alternatively, the capacitor 164 may be separately provided above thetransistor 162.

The insulating layer 152 is provided over the transistor 162 and thecapacitor 164. Further, a wiring 156 for connecting the transistor 162to another transistor is provided over the insulating layer 152.Although not illustrated in FIG. 5A, the wiring 156 is electricallyconnected to the electrode layer 142 b through an electrode formed in anopening provided in the insulating layer 150, the insulating layer 152,the gate insulting film 146, and the like. Here, the electrode ispreferably provided so as to partly overlap with at least the oxidesemiconductor layer 144 of the transistor 162.

In FIGS. 5A and 5B, the transistor 160 is provided so as to overlap withat least part of the transistor 162. The source region or the drainregion of the transistor 160 is preferably provided so as to overlapwith part of the oxide semiconductor layer 144. Further, the transistor162 and the capacitor 164 are provided so as to overlap with at leastpart of the transistor 160. For example, the conductive layer 148 b ofthe capacitor 164 is provided so as to overlap with at least part of thegate electrode 128 of the transistor 160. With such a planar layout, thearea occupied by the semiconductor device can be reduced; thus, higherintegration can be achieved.

Note that the electrical connection between the electrode layer 142 band the wiring 156 may be established by direct contact of the electrodelayer 142 b and the wiring 156 with each other or through an electrodeprovided in an insulating layer lying therebetween. Alternatively, theelectrical connection may be established through a plurality ofelectrodes.

Next, an example of a circuit configuration corresponding to FIGS. 5Aand 5B is illustrated in FIG. 5C.

In FIG. 5C, a first wiring (1st Line) is electrically connected to asource electrode of the transistor 160. A second wiring (2nd Line) iselectrically connected to a drain electrode of the transistor 160. Athird wiring (a 3rd line) and one of source or drain electrodes of thetransistor 162 are electrically connected to each other, and a fourthwiring (a 4th line) and a gate electrode of the transistor 162 areelectrically connected to each other. A gate electrode of the transistor160 and one of the source electrode and the drain electrode of thetransistor 162 are electrically connected to one electrode of thecapacitor 164. A fifth line (a 5th Line, also referred to as a wordline) and the other electrode of the capacitor 164 are electricallyconnected to each other.

The semiconductor device in FIG. 5C utilizes a characteristic in whichthe potential of the gate electrode of the transistor 160 can be held,and thus enables data writing, holding, and reading as follows.

Writing and holding of data are described. First, the potential of thefourth line is set to a potential at which the transistor 162 is turnedon, so that the transistor 162 is turned on. Accordingly, the potentialof the third line is supplied to the gate electrode of the transistor160 and the capacitor 164. That is, predetermined charge is given to thegate electrode of the transistor 160 (writing). Here, charge for supplyof a potential level or charge for supply of a different potential level(hereinafter referred to as Low level charge and High level charge) isgiven. After that, the potential of the fourth line is set to apotential at which the transistor 162 is turned off, so that thetransistor 162 is turned off. Thus, the charge given to the gateelectrode of the transistor 160 is held (storing).

Since the off-state current of the transistor 162 is extremely low, thecharge of the gate electrode of the transistor 160 is held for a longtime.

Next, reading of data is described. By supplying an appropriatepotential (reading potential) to the fifth line while a predeterminedpotential (constant potential) is supplied to the first line, thepotential of the second line varies depending on the amount of chargeheld in the gate electrode of the transistor 160. This is because ingeneral, when the transistor 160 is an n-channel transistor, an apparentthreshold voltage V_(th) _(—) _(H) in the case where a high-level chargeis given to the gate electrode of the transistor 160 is lower than anapparent threshold voltage V_(th) _(—) _(L) in the case where alow-level charge is given to the gate electrode of the transistor 160.Here, an apparent threshold voltage refers to the potential of the fifthline, which is needed to turn on the transistor 160. Thus, the potentialof the fifth wiring is set to a potential V₀ which is between V_(th)_(—) _(H) and V_(th) _(—) _(L), whereby charge given to the gateelectrode of the transistor 160 can be determined. For example, in thecase where a high-level charge is given in writing, when the potentialof the fifth wiring is set to V₀ (>V_(th) _(—) _(H)), the transistor 160is turned on. In the case where a low level charge is given in writing,even when the potential of the fifth wiring is set to V₀ (<V_(th) _(—)_(L)), the transistor 160 remains in an off state. Therefore, the storeddata can be read by the potential of the second line.

Note that in the case where memory cells are arrayed to be used, onlydata of desired memory cells needs to be read. In the case where suchreading is not performed, a potential at which the transistor 160 isturned off, that is, a potential smaller than V_(th) _(—) _(H) may begiven to the fifth wiring regardless of the state of the gate electrodeof the transistor 160. Alternatively, a potential which allows thetransistor 160 to be turned on regardless of a state of the gateelectrode, that is, a potential higher than V_(th) _(—) _(L) may beapplied to the fifth lines.

When a transistor having a channel formation region formed using anoxide semiconductor and having extremely small off-state current isapplied to the semiconductor device in this embodiment, thesemiconductor device can store data for an extremely long period. Inother words, power consumption can be adequately reduced because refreshoperation becomes unnecessary or the frequency of refresh operation canbe extremely low. Moreover, stored data can be held for a long periodeven when power is not supplied (note that a potential is preferablyfixed).

Further, in the semiconductor device described in this embodiment, highvoltage is not needed for writing data and there is no problem ofdeterioration of elements. For example, unlike a conventionalnon-volatile memory, it is not necessary to inject and extract electronsinto and from a floating gate, and thus a problem such as deteriorationof a gate insulating layer does not occur at all. In other words, thesemiconductor device according to one embodiment of the presentinvention does not have a limit on the number of times of writing whichis a problem in a conventional nonvolatile memory, and reliabilitythereof is drastically improved. Furthermore, data is written dependingon the on state and the off state of the transistor, whereby high-speedoperation can be easily realized.

Since the transistor 162 includes an oxide semiconductor layer whichcontains at least four kinds of elements of indium, gallium, zinc, andoxygen, and has a composition ratio (atomic percentage) of indium astwice or more as a composition ratio of gallium and a composition ratioof zinc, the transistor 162 can have a positive threshold voltage. Withthe transistor employed, a high-quality semiconductor device can beprovided. Further, the semiconductor device in this embodiment includesa transistor in which the threshold voltage does not easily shift evenin a long-period of use; thus, the semiconductor device can have highreliability.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 4

In this embodiment, a semiconductor device which includes the transistordescribed in Embodiment 1 or 2, which can hold stored data even when notpowered, and which does not have a limitation on the number of writecycles, and which has a structure different from the structure describedin Embodiment 3 is described with reference to FIGS. 6A and 6B and FIGS.7A to 7C. Note that the transistor 162 included in the semiconductordevice in this embodiment is the transistor described in Embodiment 1 or2. Any of the transistors described in Embodiment 1 or 2 can be used asthe transistor 162.

FIG. 6A illustrates an example of a circuit configuration of asemiconductor device, and FIG. 6B is a conceptual diagram illustratingan example of a semiconductor device. First, the semiconductor deviceillustrated in FIG. 6A is described, and then, the semiconductor deviceillustrated in FIG. 6B is described.

In the semiconductor device illustrated in FIG. 6A, a bit line BL iselectrically connected to the source electrode or the drain electrode ofthe transistor 162, a word line WL is electrically connected to the gateelectrode of the transistor 162, and the source electrode or the drainelectrode of the transistor 162 is electrically connected to a firstterminal of a capacitor 254.

The transistor 162 including an oxide semiconductor has extremely lowoff-state current. For that reason, a potential of the first terminal ofthe capacitor 254 (or a charge accumulated in the capacitor 254) can beheld for an extremely long period by turning off the transistor 162.

Next, writing and holding of data in the semiconductor device (a memorycell 250) illustrated in FIG. 6A are described.

First, the potential of the word line WL is set to a potential at whichthe transistor 162 is turned on, so that the transistor 162 is turnedon. Accordingly, the potential of the bit line BL is supplied to thefirst terminal of the capacitor 254 (writing). After that, the potentialof the word line WL is set to a potential at which the transistor 162 isturned off, so that the transistor 162 is turned off. Thus, the chargeat the first terminal of the capacitor 254 is held (holding).

Because the off-state current of the transistor 162 is extremely small,the potential of the first terminal of the capacitor 254 (or the chargeaccumulated in the capacitor) can be held for a long time.

Secondly, reading of data is described. When the transistor 162 isturned on, the bit line BL which is in a floating state and thecapacitor 254 are electrically connected to each other, and the chargeis redistributed between the bit line BL and the capacitor 254. As aresult, the potential of the bit line BL is changed. The amount ofchange in potential of the bit line BL varies depending on the potentialof the first terminal of the capacitor 254 (or the charge accumulated inthe capacitor 254).

For example, the potential of the bit line BL after chargeredistribution is (C_(B)*V_(B0)+C*V)/(C_(B)+C), where V is the potentialof the first terminal of the capacitor 254, C is the capacitance of thecapacitor 254, C_(B) is the capacitance of the bit line BL (hereinafteralso referred to as bit line capacitance), and V_(B0) is the potentialof the bit line BL before the charge redistribution. Therefore, it canbe found that assuming that the memory cell 250 is in either of twostates in which the potentials of the first terminal of the capacitor254 are V₁ and V₀ (V₁>V₀), the potential of the bit line BL in the caseof holding the potential V₁(=(C_(B)*V_(B0)+C*V₁)/(C_(B)+C)) is higherthan the potential of the bit line BL in the case of holding thepotential V₀(=(C_(B)*V_(B0)+C*V₀)/(C_(B)+C)).

Then, by comparing the potential of the bit line BL with a predeterminedpotential, data can be read.

As described above, the semiconductor device illustrated in FIG. 6A canhold charge that is accumulated in the capacitor 254 for a long timebecause the off-state current of the transistor 162 is extremely low. Inother words, power consumption can be adequately reduced because refreshoperation becomes unnecessary or the frequency of refresh operation canbe extremely low. Moreover, stored data can be stored for a long timeeven when power is not supplied.

Next, the semiconductor device illustrated in FIG. 6B is described.

The semiconductor device illustrated in FIG. 6B includes a memory cellarray 251 (memory cell arrays 251 a and 251 b) including a plurality ofmemory cells 250 illustrated in FIG. 6A as memory circuits in the upperportion, and a peripheral circuit 253 in the lower portion which isnecessary for operating the memory cell array 251 (the memory cellarrays 251 a and 251 b). Note that the peripheral circuit 253 iselectrically connected to the memory cell array 251.

In the structure illustrated in FIG. 6B, the peripheral circuit 253 canbe provided under the memory cell array 251 (the memory cell arrays 251a and 251 b). Thus, the size of the semiconductor device can bedecreased.

It is preferable that a semiconductor material of the transistorprovided in the peripheral circuit 253 be different from that of thetransistor 162. For example, silicon, germanium, silicon germanium,silicon carbide, gallium arsenide, or the like can be used, and a singlecrystal semiconductor is preferably used. Alternatively, an organicsemiconductor material or the like may be used. A transistor includingsuch a semiconductor material can operate at sufficiently high speed.Therefore, a variety of circuits (e.g., a logic circuit or a drivercircuit) which needs to operate at high speed can be favorably realizedby the transistor.

Note that FIG. 6B illustrates, as an example, the semiconductor devicein which two memory cell arrays (the memory cell array 251 a and thememory cell array 251 b) are stacked; however, the number of memory cellarrays to be stacked is not limited thereto. Three or more memory cellarrays may be stacked.

Next, a specific structure of the memory cell 250 illustrated in FIG. 6Ais described with reference to FIGS. 7A to 7C.

FIGS. 7A to 7C illustrate a structure example of the memory cell 250.FIG. 7A is a plan view of the memory cell 250. FIG. 7B is across-sectional view taken along line A-B in FIG. 7A.

The transistor 162 in FIGS. 7A and 7B can have the same structure as thetransistor in Embodiment 1 or 2.

As illustrated in FIG. 7B, the transistor 162 is formed over anelectrode 502 and an electrode 504. The electrode 502 serves as a bitline BL in FIG. 6A and is in contact with the low-resistance region ofthe transistor 162. The electrode 504 serves as one electrode of thecapacitor 254 in FIG. 6A and is in contact with the low-resistanceregion of the transistor 162. Over the transistor 162, the electrode 506provided in a region overlapping with the electrode 504 serves as theother electrode of the capacitor 254.

As illustrated in FIG. 7A, the other electrode 506 of the capacitor 254is electrically connected to a capacitor line 508. A gate electrode 148a over the oxide semiconductor layer 144 with the gate insulating film146 provided therebetween is electrically connected to a word line 509.

FIG. 7C is a cross-sectional view in a connection portion between thememory cell array 251 and the peripheral circuit. The peripheral circuitcan include, for example, an n-channel transistor 510 and a p-channeltransistor 512. The n-channel transistor 510 and the p-channeltransistor 512 are preferably formed using a semiconductor materialother than an oxide semiconductor (e.g., silicon). With such a material,the transistor included in the peripheral circuit can operate at highspeed.

When the planar layout in FIG. 7A is employed, the area occupied by thesemiconductor device can be reduced; thus, the degree of integration canbe increased.

As described above, the plurality of memory cells formed in multiplelayers in the upper portion each include a transistor including an oxidesemiconductor. Since the off-state current of the transistor includingan oxide semiconductor which contains at least four kinds of elements ofindium, gallium, zinc, and oxygen, and has a composition ratio (atomicpercentage) of indium as twice or more as a composition ratio of galliumand a composition ratio of zinc, is low, stored data can be held for along time owing to the transistor. In other words, the frequency ofrefresh operation can be significantly lowered, which leads to asufficient reduction in power consumption. Further, as illustrated inFIG. 7B, the capacitor 254 is formed by stacking the electrode 504, theoxide semiconductor layer 144, the gate insulating film 146, and theelectrode 506. Since the relative permittivity of the oxidesemiconductor layer with the above-described composition is extremelyhigh (a relative permittivity of 66), the area required for thecapacitor 254 can be reduced when the oxide semiconductor layer is usedas a dielectric film.

A semiconductor device having a novel feature can be obtained by beingprovided with both a peripheral circuit including the transistorincluding a material other than an oxide semiconductor (in other words,a transistor capable of operating at sufficiently high speed) and amemory circuit including the transistor including an oxide semiconductor(in a broader sense, a transistor whose off-state current issufficiently small). In addition, with a structure where the peripheralcircuit and the memory circuit are stacked, the degree of integration ofthe semiconductor device can be increased.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 5

In this embodiment, examples of application of the semiconductor devicedescribed in any of the above embodiments to portable devices such ascellular phones, smartphones, or electronic books will be described withreference to FIGS. 8A and 8B, FIG. 9, FIG. 10, and FIG. 11.

In portable electronic devices such as a mobile phone, a smart phone,and an e-book reader, an SRAM or a DRAM is used so as to store imagedata temporarily. This is because response speed of a flash memory islow and thus a flash memory is not suitable for image processing. On theother hand, an SRAM or a DRAM has the following characteristics whenused for temporary storage of image data.

In an ordinary SRAM, as illustrated in FIG. 8A, one memory cell includessix transistors, that is, transistors 801 to 806, which are driven withan X decoder 807 and a Y decoder 808. The transistors 803 and 805 andthe transistors 804 and 806 each serve as an inverter, and high-speeddriving can be performed therewith. However, an SRAM has a disadvantageof large cell area because one memory cell includes six transistors.Provided that the minimum feature size of a design rule is F, the areaof a memory cell in an SRAM is generally 100 F² to 150 F². Therefore, aprice per bit of an SRAM is the most expensive among a variety of memorydevices.

In a DRAM, as illustrated in FIG. 8B, a memory cell includes atransistor 811 and a storage capacitor 812, which are driven with an Xdecoder 813 and a Y decoder 814. One cell includes one transistor andone capacitor and thus the area of a memory cell is small. The area of amemory cell of a DRAM is generally less than or equal to 10 F². Notethat in the case of a DRAM, a refresh operation is always necessary andpower is consumed even when a rewriting operation is not performed.

However, the area of the memory cell of the semiconductor devicedescribed the above embodiments is about 10 F² and frequent refreshingis not needed. Therefore, the area of the memory cell is reduced, andthe power consumption can be reduced.

Next, FIG. 9 is a block diagram of a portable device. The portabledevice illustrated in FIG. 9 includes an RF circuit 901, an analogbaseband circuit 902, a digital baseband circuit 903, a battery 904, apower supply circuit 905, an application processor 906, a flash memory910, a display controller 911, a memory circuit 912, a display 913, atouch sensor 919, an audio circuit 917, a keyboard 918, and the like.The display 913 includes a display portion 914, a source driver 915, anda gate driver 916. The display 913 includes a display portion 914, asource driver 915, and a gate driver 916. The application processor 906includes a CPU 907, a DSP 908, and an interface 909 (IF 909). Ingeneral, the memory circuit 912 includes an SRAM or a DRAM; by employingthe semiconductor device described in any of the above embodiments forthe memory circuit 912, writing and reading of data can be performed athigh speed, data can be held for a long time, and power consumption canbe sufficiently reduced.

FIG. 10 illustrates an example of using the semiconductor devicedescribed in any of the above embodiments in a memory circuit 950 for adisplay. The memory circuit 950 illustrated in FIG. 10 includes a memory952, a memory 953, a switch 954, a switch 955, and a memory controller951. Further, in the memory circuit 950, a signal line (input imagedata), a display controller 956 which reads and controls data held inthe memories 952 and 953, and a display 957 which displays data by asignal from the display controller 956 are connected.

First, image data (input image data A) is formed by an applicationprocessor (not shown). The input image data A is held in the memory 952though the switch 954. The image data (stored image data A) held in thememory 952 is transmitted and displayed to the display 957 through theswitch 955 and the display controller 956.

In the case where the input image data A is not changed, the storedimage data A is read from the display controller 956 through the memory952 and the switch 955 with a frequency of 30 Hz to 60 Hz in general.

Next, for example, when data displayed on the screen is rewritten by auser (that is, in the case where the input image data A is changed), newimage data (input image data B) is formed by the application processor.The input image data B is held in the memory 953 through the switch 954.The stored image data A is read periodically from the memory 952 throughthe switch 955 even during that time. After the completion of storingthe new image data (the stored image data B) in the memory 953, from thenext frame for the display 957, the stored image data B starts to beread, transmitted to the display 957 through the switch 955 and thedisplay controller 956, and displayed on the display 957. This readingoperation is continued until another new image data is held in thememory 952.

By alternately writing and reading image data to and from the memory 952and the memory 953 as described above, images are displayed on thedisplay 957. Note that the memory 952 and the memory 953 are not limitedto separate memories, and a single memory may be divided and used. Byemploying the semiconductor device described in any of the aboveembodiments for the memory 952 and the memory 953, data can be writtenand read at high speed and held for a long time, and power consumptioncan be sufficiently reduced.

FIG. 11 is a block diagram of an electronic book. FIG. 11 includes abattery 1001, a power supply circuit 1002, a microprocessor 1003, aflash memory 1004, an audio circuit 1005, a keyboard 1006, a memorycircuit 1007, a touch panel 1008, a display 1009, and a displaycontroller 1010.

Here, the semiconductor device described in any of the above embodimentscan be used for the memory circuit 1007 in FIG. 11. The memory circuit1007 has a function of temporarily storing the contents of a book. Forexample, users use a highlight function in some cases. When the userreads an e-book, the user will put a mark on a specific part in somecases. Such a marking function is called a highlighting function, bywhich characters are changed in color or type, underlined, orbold-faced, for example, so that a specific part is made to lookdistinct from the other part. In the function, information about thepart specified by the user is stored and retained. In the case where theinformation is stored for a long time, the information may be copied tothe flash memory 1004. Even in such a case, by employing thesemiconductor device described in any of the above embodiments, writingand reading of data can be performed at high speed, data can be held fora long time, and power consumption can be sufficiently reduced.

As described above, the semiconductor device in any of the aboveembodiments is mounted on each of the portable devices described in thisembodiment. Therefore, a portable electric device in which writing andreading of data are performed at high speed, data is held for a longtime, and power consumption is sufficiently reduced, can be obtained.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the other structures, methods,and the like described in the other embodiments.

Example 1

In this example, an oxide semiconductor film (IGZO film) containingindium, gallium, and zinc was formed, ionization potentials of the oxidesemiconductor film were measured, and an energy band diagram wasgenerated according to the measurement results. In this specification,the level of the ionization potential corresponds to the sum of the bandgap (energy gap) and the electron affinity, and the value of the bandgap is a value obtained by measuring a single material film byspectroscopic ellipsometry. In addition, composition analysis wasperformed on the oxide semiconductor film.

First, the values of the band gap measured by spectroscopic ellipsometryare shown.

As a sample oxide semiconductor film, a 100-nm-thick IGZO film wasformed over a quartz substrate by a sputtering method. For thedeposition conditions, the substrate temperature was 300° C., and anoxide target having a composition ratio of In:Ga:Zn=3:1:2 [atomic ratio]was used.

The band gaps of the samples were about 2.8 eV to 2.9 eV: when a samplewas deposited in an argon and oxygen atmosphere (argon:oxygen=30 sccm:15 sccm) and heat treatment after the deposition was not performed, aband gap was 2.83 eV; when a sample was deposited in an argon and oxygenatmosphere (argon:oxygen=30 sccm: 15 sccm) and heat treatment wasperformed at 450° C. (in a nitrogen atmosphere for one hour and then inan oxygen atmosphere for one hour) after the deposition, a band gap was2.90 eV; when a sample was deposited in an argon and oxygen atmosphere(argon:oxygen=30 sccm: 15 sccm) and heat treatment was performed at 650°C. (in a nitrogen atmosphere for one hour and then in an oxygenatmosphere for one hour) after the deposition, a band gap was 2.94 eV;when a sample was deposited in an oxygen atmosphere (the proportion ofoxygen in the atmosphere is 100%) and heat treatment was not performedafter the deposition, a band gap was 2.82 eV; when a sample wasdeposited in an oxygen atmosphere (the proportion of oxygen in theatmosphere is 100%) and heat treatment was performed at 450° C. (in anitrogen atmosphere for one hour and then in an oxygen atmosphere forone hour) after the deposition, a band gap was 2.89 eV; when a samplewas deposited in an oxygen atmosphere (the proportion of oxygen in theatmosphere is 100%) and heat treatment was performed at 650° C. (in anitrogen atmosphere for one hour and then in an oxygen atmosphere forone hour) after the deposition, a band gap was 2.94 eV.

In addition, an IGZO film was deposited to a thickness of 15 nm byirradiating ultraviolet light from a surface side over a single-crystalsilicon substrate at a substrate temperature of 300° C. in an oxygenatmosphere (the proportion of oxygen in the atmosphere is 100%) with theuse of an oxide target having a composition ratio of In:Ga:Zn=3:1:2[atomic ratio] and the ionization potential was measured by ultravioletphotoelectron spectroscopy (UPS) while the surface of the IGZO film isirradiated by ultraviolet light. Note that the ionization potentialcorresponds to an energy difference between a vacuum level and a valenceband.

The energy of the conduction band was obtained by subtracting the bandgap measured by spectroscopic ellipsometry from the value of theionization potential, and the band structure of the IGZO film which wasdeposited with the use of an oxide target having a composition ratio ofIn:Ga:Zn=3:1:2 [atomic ratio] was formed. Note that the band gap of theIGZO film was 2.8 eV. FIG. 12 shows the band gap.

Next, the composition of the IGZO film which was deposited to athickness of 15 nm over a single-crystal silicon substrate by sputteringat a substrate temperature of 300° C. in an oxygen atmosphere (theproportion of oxygen in the atmosphere is 100%) with the use of an oxidetarget having a composition ratio of In:Ga:Zn=3:1:2 [atomic ratio], wasevaluated by being quantified by X-ray photoelectron spectroscopy (XPS)analysis.

The IGZO film contained a 23.7 atomic % of indium (In), a 7.5 atomic %of gallium (Ga), a 9 atomic % of zinc (Zn), and a 59.7 atomic % ofoxygen (O).

Further, X-ray diffraction (XRD) measurement was performed on the IGZOfilm deposited with the use of an oxide target having a compositionratio of In:Ga:Zn=3:1:2 [atomic ratio].

As a sample, a 100-nm-thick IGZO film was formed over a quartz substrateby a sputtering method. Deposition conditions were as follows: asubstrate temperature of room temperature, 200° C., 300° C., or 400° C.,an atmosphere of argon and oxygen (argon:oxygen=30 sccm: 15 sccm), andan oxide target having a composition ratio of In:Ga:Zn=3:1:2 [atomicratio].

FIG. 13 shows measurement results of the out-of-plane XRD spectra of theIGZO films. In FIG. 13, the vertical axis indicates the X-raydiffraction intensity (arbitrary unit) and the horizontal axis indicatesthe rotation angle 2θ (degree). Note that the XRD spectra were measuredwith an X-ray diffractometer D8 ADVANCE manufactured by Bruker AXS K.K.

In the IGZO film deposited at a room temperature, a peak indicatingcrystallinity was not observed in the XRD spectrum as shown in FIG. 13,that is, it was confirmed that the IGZO film is an amorphous oxidesemiconductor film. In addition, in each of the IGZO films deposited at200° C., 300° C., or 400° C., a peak attributed to crystallinity wasobserved at around 31° (=2θ) in the XRD spectrum as shown in FIG. 13,that is, it was confirmed that the IGZO films are crystalline oxidesemiconductor films.

End planes of the IGZO films were cut out, and cross sections of theIGZO films thereof were observed with a high resolution transmissionelectron microscope (TEM) (“H9000-NAR” manufactured by HitachiHigh-Technologies Corporation) at an acceleration voltage of 300 kV.

As a sample, a 100-nm-thick IGZO film was formed over a quartz substrateby a sputtering method. Deposition conditions were as follows: asubstrate temperature of 300° C., an atmosphere of argon and oxygen(argon:oxygen=30 sccm: 15 sccm), and an oxide target having acomposition ratio of In:Ga:Zn=3:1:2 [atomic ratio].

FIG. 16A is a cross-sectional TEM image of an IGZO film which is notsubjected to heat treatment after deposition. FIG. 16B is across-sectional TEM image of an IGZO film which is subjected to heattreatment at 450° C. (in a nitrogen atmosphere for one hour and then inan oxygen atmosphere for one hour) after deposition. FIG. 16C is across-sectional TEM image of an IGZO film which is subjected to heattreatment at 650° C. (in a nitrogen atmosphere for one hour and then inan oxygen atmosphere for one hour) after deposition.

As shown in FIGS. 16A to 16C, crystals having a c-axis substantiallyperpendicular to a surface (CAAC) are confirmed in these IGZO films.

As described above, it was confirmed that a non-single crystal IGZO filmis obtained with the use of an oxide target having a composition ratioof In:Ga:Zn=3:1:2 [atomic ratio].

Example 2

In this example, a transistor including an IGZO film formed with anoxide target having a composition ratio of In:Ga:Zn=3:1:2 [atomic ratio]was manufactured and electrical characteristics and reliability of thetransistor were evaluated.

As the transistor, a transistor 1 having the structure of the transistor440 a in FIGS. 1A to 1E and a transistor 2 having the structure of thetransistor 440 b in FIG. 2A were manufactured. A method formanufacturing the transistor 1 and the transistor 2 is described below.

As an insulating layer, a 300-nm-thick silicon oxide film was depositedover a glass substrate by a sputtering method (deposition conditions: anoxygen atmosphere, a pressure of 0.4 Pa, a power of 1.5 kW, a distancebetween the glass substrate and a target of 60 mm, and a substratetemperature of 100° C.).

The surface of the silicon oxide film was polished and then, a20-nm-thick IGZO film was deposited as an oxide semiconductor film, by asputtering method with the use of an oxide target having a compositionratio of In:Ga:Zn=3:1:2 [atomic ratio]. Deposition conditions were asfollows: an atmosphere of argon and oxygen (argon:oxygen=30 sccm: 15sccm), a pressure of 0.4 Pa, a power of 1.5 kW, a distance between theglass substrate and the target of 60 mm, and a substrate temperature of200° C.

Then, heat treatment was performed at 450° C. in a nitrogen atmospherefor one hour and then in an oxygen atmosphere for one hour. The IGZOfilm was processed into an island shape by an inductively coupled plasmaetching (etching conditions: an etching gas of BCl₃:Cl₂=60 sccm: 20sccm, a power of 450 W, a bias power of 100 W, and a pressure of 1.9Pa).

A 50-nm-thick tungsten film was deposited by sputtering (depositionconditions: an argon atmosphere, a pressure of 0.8 Pa, and a power of 1kW) and was etched (etching conditions: an etching gas of CF₄:Cl₂:O₂=25sccm: 25 sccm: 10 sccm, a power of 500 W, a bias power of 150 W, and apressure of 1.0 Pa) to form a source electrode layer and a drainelectrode layer.

Next, a 30-nm-thick silicon oxynitride film was deposited as a gateinsulating film by a CVD method.

A stack of a 15-nm-thick tantalum nitride film (deposition conditions:an atmosphere of argon and nitrogen (Ar:N₂=50 sccm: 10 sccm), a pressureof 0.6 Pa, and a power of 1 kW) and a 135-nm-thick tungsten film(deposition conditions: an argon atmosphere, a pressure of 2.0 Pa, and apower of 4 kW) was formed by a sputtering, and was etched (first etchingconditions: an etching gas of Cl₂:SF₆:O₂=33 sccm: 33 sccm: 10 sccm, apower of 2000 W, a bias power of 50 W, and a pressure of 0.67 Pa; andsecond etching conditions: Cl₂=100 sccm, a power of 2000 W, a bias powerof 50 W, and a pressure of 0.67 Pa), so that a gate electrode layer wasformed.

Ion implantation of phosphorus (P) was performed on the IGZO film of thetransistor 1 with the use of the gate electrode layer, the sourceelectrode layer, and the drain electrode layer as masks. Note that theconditions of the phosphorus (P) ion implantation were as follows: anacceleration voltage of 40 kV and a dosage of 1.0×10¹⁵ ions/cm².

As an insulating film, an aluminum oxide film was deposited over thegate electrode layer, by sputtering (deposition conditions: anatmosphere of argon and oxygen (argon:oxygen=25 sccm: 25 sccm), apressure of 0.4 Pa, a power of 2.5 kW, a distance between the glasssubstrate and the target of 60 mm, and a substrate temperature of 250°C.). Then, a 300-nm-thick silicon oxynitride film was stacked over thealuminum oxide film by a CVD method.

Next, an opening reaching the IGZO film was formed in the gateinsulating film and the insulating film, and a stack of a 50-nm-thicktitanium film (deposition conditions: an argon atmosphere (Ar=20 sccm),a pressure of 0.1 Pa, a power of 12 kW), a 100-nm-thick aluminum film(deposition conditions: an argon atmosphere (Ar=50 sccm), a pressure of0.4 Pa, a power of 1 kW), and a 50-nm-thick titanium film (depositionconditions: an argon atmosphere (Ar=20 sccm), a pressure of 0.1 Pa, apower of 12 kW) was formed in the opening and was etched (etchingconditions: an etching gas of BCl₃:Cl₂=60 sccm: 20 sccm, a power of 450W, a bias power of 100 W, and a pressure of 1.9 Pa), so that a wiringlayer was formed.

Through the above process, the transistor 1 and the transistor 2 weremanufactured. Note that in the transistor 1, the channel length (L) was3.2 μm, the channel width (W) was 10.1 μm, and the width in the channellength direction (also referred to as Loft) of a region which does notoverlap with the source electrode layer, the drain electrode layer, andthe gate electrode layer over the oxide semiconductor film, was 0.15 μm.In the transistor 2, the channel length (L) was 2.9 μm, the channelwidth (W) was 10.1 μm, and the width in the channel length direction(also referred to as Lov) of a region where the source electrode layeror the drain electrode layer overlaps with the gate electrode layer overthe oxide semiconductor film, was 1.15 μl.

Electrical characteristics of the transistors 1 and 2 and reliability ofthe transistor 1 were evaluated. FIG. 14 shows gate voltage(V_(g))-drain current (I_(d)) characteristics of the transistor 2 whendrain voltages (V_(d)) are 3 V and 0.1 V and field-effect mobility whenthe drain voltage (V_(d)) is 0.1V. FIGS. 15A and 15B show gate voltage(V_(g))-drain current (I_(d)) characteristics of the transistor 1 whendrain voltages (V_(d)) are 3 V and 0.1 V and field-effect mobility whenthe drain voltage (V_(d)) is 0.1V.

As shown in FIG. 14 and FIGS. 15A and 15B, the field-effect mobility wasapproximately 20 cm²/Vs, particularly the field-effect mobility of thetransistor 2 was over 20 cm²/Vs, which shows that the transistors 1 and2 have excellent ON characteristics.

One of methods for examining reliability of transistors is abias-temperature stress test (hereinafter, referred to as a gate biastemperature (GBT) test). The GBT test is one kind of accelerated testand a change in characteristics, caused by long-term use, of transistorscan be evaluated in a short time. In particular, the amount of shift inthreshold voltage of the transistor between before and after a GBT testis an important indicator for examining reliability. The smaller theshift in the threshold voltage between before and after a GBT test is,the higher the reliability of the transistor is.

The temperature of a substrate over which a transistor is formed is setat a fixed temperature. A source and a drain of the transistor are setat the same potential, and a gate is supplied with a potential differentfrom those of the source and the drain for a certain period. Thetemperature of the substrate may be determined depending on the purposeof the test. Further, the potential applied to the gate is higher thanthe potential of the source and the drain (the potential of the sourceand the drain is the same) in a “+GBT test” while the potential appliedto the gate is lower than the potential of the source and the drain (thepotential of the source and the drain is the same) in a “−GBT test.”

Strength of the GBT test may be determined based on the temperature of asubstrate and electric field intensity and time period of application ofthe electric field to a gate insulating layer. The electric fieldintensity in the gate insulating layer is determined as the value of apotential difference between a gate, and a source and a drain divided bythe value of the thickness of the gate insulating layer.

In this example, the GBT test was performed on the transistor 1. First,as a +GBT test, V_(g)-I_(d) characteristics of the transistor 1 weremeasured at a substrate temperature of 40° C. and a drain voltage V_(d)of 3 V. Then, the substrate temperature was set to 150° C. and V_(d) wasset to 0.1 V. After that, a gate voltage V_(g) of 6 V was applied sothat the intensity of an electric field applied to the gate insulatingfilm was 2 MV/cm, and the condition was kept for one hour in an air.Next, V_(g) was set to 0 V. Then, V_(g)-I_(d) characteristics of thetransistor 1 were measured at a substrate temperature of 40° C. andV_(d) of 10 V. FIG. 15A shows results of the +GBT test.

Similarly, V_(g)-I_(d) characteristics of the transistor 1 were measuredat a substrate temperature of 40° C. and V_(d) of 10 V. Then, thesubstrate temperature was set to 150° C. and V_(d) was set to 0.1 V.After that, V_(g) of −6 V was applied so that the intensity of anelectric field applied to the gate insulating film was −2 MV/cm, and thecondition was kept for one hour in an air atmosphere. Next, V_(g) wasset to 0 V. Then, V_(g)-I_(d) characteristics of the transistor 1 weremeasured at a substrate temperature of 40° C. and V_(d) of 10 V. FIG.15B shows results of the −GBT test.

Note that in FIGS. 15A and 15B, a thick line represents results before aGBT test and a thin line represents results after the GBT test.

As shown in FIGS. 15A and 15B, the transistor 1 shows substantially nochange in the threshold voltage through the +GBT test and the −GBT test.Thus, it was confirmed that the amount of changes in the thresholdvoltage through the +GBT test and the −GBT test is small and reliabilityis high in the transistor in this example.

Further, a transistor in which a channel length (L) was 0.8 μm, achannel width (W) was 1000 μm, and an Loff was 0.3 μm was manufacturedby a manufacturing process similar to that of the transistor 1, and theoff-state leakage current (off-state current) of the transistor wasmeasured. The measurement was performed at 125° C. and 85° C. Themeasurement results are shown in FIG. 17.

As shown in FIG. 17, the off-leakage current when the transistor in thisexample was operated at 85° C. for 41.5 hours was 0.5 zA/μm, which isextremely low.

As described above, it was confirmed that the transistor in this examplehas an extremely low off-state current and has high reliability.

This application is based on Japanese Patent Application serial No.2011-161383 filed with Japan Patent Office on Jul. 22, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: an oxidesemiconductor layer including a channel formation region, a firstregion, and a second region; a source electrode layer and a drainelectrode layer over the oxide semiconductor layer; and a gate electrodelayer over the oxide semiconductor layer with a gate insulating filminterposed therebetween, wherein the first region is not overlapped withthe gate electrode layer, the source electrode layer and the drainelectrode layer, wherein the second region is overlapped with the sourceelectrode layer or the drain electrode layer, and wherein a resistancein the second region is higher than a resistance in the first region. 2.The semiconductor device according to claim 1, wherein the first regionand the second region include a dopant, and wherein a dopantconcentration in the second region is lower than a dopant concentrationin the first region.
 3. The semiconductor device according to claim 1,wherein the oxide semiconductor layer includes a c-axis aligned crystalregion.
 4. The semiconductor device according to claim 1, wherein theoxide semiconductor layer is formed with an oxide target having acomposition ratio of indium:gallium:zinc=3:1:2.
 5. The semiconductordevice according to claim 1, wherein a sidewall insulating layer isprovided on a side surface of the gate electrode layer.
 6. Thesemiconductor device according to claim 1, further comprising atransistor including a channel formation region comprising silicon. 7.An electronic device comprising the semiconductor device according toclaim 1, wherein the electronic device includes, as a component, an LSI;a CPU; a power device mounted in a power circuit; a semiconductorintegrated circuit including a memory, a thyristor, a converter, or animage sensor; an electro-optical device typified by a liquid crystaldisplay panel; or a light-emitting display device including alight-emitting element.
 8. The semiconductor device according to claim1, wherein the oxide semiconductor layer contains at least four kinds ofelements of indium, gallium, zinc, and oxygen, and a composition ratioof indium is twice or more that of gallium and that of zinc, and whereinthe composition ratio is represented by atomic percentage.
 9. Asemiconductor device comprising: a pair of first oxide semiconductorlayers separated from each other; a second oxide semiconductor layerincluding a channel formation region over and in contact with the pairof first oxide semiconductor layers; a gate electrode layer over thesecond oxide semiconductor layer with a gate insulating film interposedtherebetween; a source electrode layer overlapping with one of the pairof first oxide semiconductor layers and the second oxide semiconductorlayer; and a drain electrode layer overlapping with the other of thepair of first oxide semiconductor layers and the second oxidesemiconductor layer; wherein the source electrode layer and the drainelectrode layer are in contact with the second oxide semiconductorlayer.
 10. The semiconductor device according to claim 9, wherein thepair of first oxide semiconductor layers and a region of the secondoxide semiconductor layer which does not overlap with the gate electrodelayer include a dopant.
 11. The semiconductor device according to claim9, wherein the second oxide semiconductor layer includes a c-axisaligned crystal region.
 12. The semiconductor device according to claim9, wherein the second oxide semiconductor layer is formed with an oxidetarget having a composition ratio of indium:gallium:zinc=3:1:2.
 13. Thesemiconductor device according to claim 9, wherein a sidewall insulatinglayer is provided on a side surface of the gate electrode layer.
 14. Thesemiconductor device according to claim 9, further comprising atransistor including a channel formation region comprising silicon. 15.An electronic device comprising the semiconductor device according toclaim 9, wherein the electronic device includes, as a component, an LSI;a CPU; a power device mounted in a power circuit; a semiconductorintegrated circuit including a memory, a thyristor, a converter, or animage sensor; an electro-optical device typified by a liquid crystaldisplay panel; or a light-emitting display device including alight-emitting element.
 16. The semiconductor device according to claim9, wherein the second oxide semiconductor layer contains at least fourkinds of elements of indium, gallium, zinc, and oxygen, and acomposition ratio of indium is twice or more that of gallium and that ofzinc, and wherein the composition ratio is represented by atomicpercentage.
 17. The semiconductor device according to claim 16, whereinthe pair of first oxide semiconductor layers is formed with an oxidetarget having a composition ratio of indium:gallium:zinc=1:1:1.